What's the best way to clean everything off a hard drive
before I return it to Dell?

Will a low-level format do it or is there something better>
The drive has five partitions.

Thanks!

On 11/27/23 07:45, jason_warren@ieee.org wrote:
> What's the best way to clean everything off a hard drive
> before I return it to Dell?
>
> Will a low-level format do it or is there something better>
> The drive has five partitions.

Maybe, you will have to look at the description of the low
level format. Me thinks it is not wiping, but testing.
(Testing removes bad sectors from the file table.) Paul
will know the particulars of a particular low level format.

>
> Thanks!
>

Does Dell expect the hard drive to be populated
when you return it?

If so, I'd create a new use user account and
erase your old one, in the process erasing everything
that was yours. Then run it through Bleach Bit's
erase unused space option.

https://www.bleachbit.org/download/windows

~~~~~~~~

If Dell does not expect a drive to come back. I'd just
remove it. I do this all the time for customers as
vendors do not repair, they replace the computer. So you
get back a new factory computer. When the unit comes back,
I reinstall the old hard drive and now they have a
spare drive too!

~~~~~~~~

If you need to blank out the drive before returning it, dd
to the rescue. Download a Linux live ISO. Here are two
good ones:

https://fedoraproject.org/spins/xfce/download
https://fedoraproject.org/spins/mate/download

You can cut them to a USB flash drive with Rufus
or Fedora Media writer:

https://rufus.akeo.ie/
https://fedoraproject.org/en/workstation/download

Boot off the Live USB, open a terminal (right click on the
desktop), elevate to root with "su", the run a

lf -kPT

to locate your hard drive. The size of the drive will be the
give away. It will be something like /dev/sdba (the "a" is
the partition) if a spinning drive or a SATA SSD or
something like /dev/nvme0n1p1 (the p1 is the partition).

Then fire up dd:
dd bs=1M if=/dev/zero of=[your drive without the partition]

~~~~~~~~

And if you just want to destroy the drive, remove and it and
whack it good several times with a hammer. Wear safety goggles.

Hope that all helps,
-T

On 2023-11-27 16:45, jason_warren@ieee.org wrote:
> What's the best way to clean everything off a hard drive
> before I return it to Dell?
>
> Will a low-level format do it or is there something better>
> The drive has five partitions.

Low level format disappeared ages ago.

The methods for complete erasure vary, but you can always ask the
firmware to do a secure erase of the ATA standard.

On Linux:

man hdparm

ATA Security Feature Set

--security-erase PWD
Erase (locked) drive, using password PWD (DANGER-
OUS). Password is given as an ASCII string and is
padded with NULs to reach 32 bytes. Use the spe-
cial password NULL to represent an empty password.
The applicable drive password is selected with the
--user-master switch (default is "user" password).
No other options are permitted on the command line
with this one.

--security-erase-enhanced PWD
Enhanced erase (locked) drive, using password PWD
(DANGEROUS). Password is given as an ASCII string
and is padded with NULs to reach 32 bytes. The ap-
plicable drive password is selected with the
--user-master switch (default is "user" password).
No other options are permitted on the command line
with this one.

On Windows, you will need some program to do it.

It is possible your BIOS/UEFI has a function to do it.

If your disk is not "rotating rust", don't try any of the old programs
that fill the entire disk with zeroes or other patterns several times:
it kill lives of SSD/NVME media and is useless.

--
Cheers,
Carlos E.R.

On 11/27/23 09:33, T wrote:
> If so, I'd create a new use user account and
> erase your old one, in the process erasing everything
> that was yours.  Then run it through Bleach Bit's
> erase unused space option.
>
> https://www.bleachbit.org/download/windows

Oops, dump your trash bin first!

On 11/27/23 09:42, Carlos E. R. wrote:
>
> Low level format disappeared ages ago.

Hi Carlos,

I had presumes that if you took off the quick format
option, that it did a low level format. It does
take its sweet time!

https://imgur.com/zc3T9dO.png

Am I missing something?

-T

On 2023-11-27 19:03, T wrote:
> On 11/27/23 09:42, Carlos E. R. wrote:
>>
>> Low level format disappeared ages ago.
>
>
> Hi Carlos,
>
> I had presumes that if you took off the quick format
> option, that it did a low level format.  It does
> take its sweet time!
>
> https://imgur.com/zc3T9dO.png
>
> Am I missing something?

A low level format creates the markings that allow the firmware to know
where each track and sector on it is. Before that, the plates are empty,
no magnetic signals. Ie, it only applies to magnetic rotating media.
maybe some optical media, dunno. Only factory can do it AFAIK.

You are talking of "full format".

On the original IBM PC and clones, which used hard disks with stepper
motors, the low level format was accessed from the debug command, to run
directly a routine in the firmware.

--
Cheers,
Carlos E.R.

On 11/27/23 10:12, Carlos E. R. wrote:
> On the original IBM PC and clones, which used hard disks with stepper
> motors, the low level format was accessed from the debug command, to run
> directly a routine in the firmware.

Now that was a trip down memory lane. :-)

jason_warren@ieee.org wrote:
> What's the best way to clean everything off a hard drive
> before I return it to Dell?
>
> Will a low-level format do it or is there something better>
> The drive has five partitions.
>
> Thanks!
>

Just do a factory reset.
Settings, System, Recovery, Reset PC, Remove Everything, and make sure
you select the Clean the drive option to ensure everything gets deleted.

Oh, and don't listen too closely to the discussions of Win10 chummies.
They're far more interested in the internal philosophy of matters
concerned than the practical task you've raised.

Ed

T <T@invalid.invalid> wrote:

> On 11/27/23 09:42, Carlos E. R. wrote:
>>
>> Low level format disappeared ages ago.
>
> Hi Carlos,
>
> I had presumes that if you took off the quick format
> option, that it did a low level format. It does
> take its sweet time!
>
> https://imgur.com/zc3T9dO.png
>
> Am I missing something?
>
> -T

Formatting does not wipe unallocated sectors, only those that have
allocated in the MFT within a partition. That means data still resides
in the unallocated sectors that had previously been allocated and
written. Also, changing partition size, like reducing it, will leave
behind sectors that might've contained data but are no longer part of a
partition for the normal wipe tools to erase.

"jason_warren@ieee.org" <jason_warren@ieee.org> wrote:

> What's the best way to clean everything off a hard drive
> before I return it to Dell?
>
> Will a low-level format do it or is there something better>
> The drive has five partitions.

I'm confused. If you are returning a hard drive to Dell, doesn't that
mean it is defective, and why you are returning it? If the drive is
defective, likely you cannot completely wipe the drive.

There are lots of wipe tools. Example: killdisk, which you can run from
a bootable disc to eliminate an OS on the drive or any access to it from
an OS to prevent locked files.

https://www.killdisk.com/eraser.html

If no part of the active OS is using the hard drive, you can use other
wipe tools that run under that OS, like CCleaner's Drive Wiper or
Heidi's Eraser.

https://www.ccleaner.com/
https://eraser.heidi.ie/

As I recall, Eraser will allow multiple means of wiping the drive.
Writing zeros, then ones, and then zeros is more than sufficient. The
Gutmann method was used way back when RLL was used on hard drives to
ensure any residual magnetism was eradicated, but is a useless wipe
scheme on newer drives.

Few drives provide low-level formatting. That was removed long ago,
because the vast majority of users didn't know what they were doing, and
ended up destroying usability thereafter. Now low-level format (LLF) is
is also called zero-fill since all zeroes are written to the drive.
Partitionining is irrelevant since those will be lost on a drive wipe.
LLF will remove all data, partitioning, boot sectors, file formats,
district DATA, identification ID, and everything on the drive. I don't
know if the bad sector map created at the factory also gets erased. If
so, you have to thoroughly test the LLF wiped drive to retest all the
sectors to remap the ones that are bad.

https://dban.org/
https://hddguru.com/software/HDD-LLF-Low-Level-Format-Tool/

Some drives have a firmware-based wipe function (ATA Secure Erase or
NVMe Secure Erase), but you need a tool that sends the command to the
drive. Often the tool is proprietary and brand specific, but the
operation is performed within the firmware on the drive.

https://www.pcworld.com/article/461014/how-to-securely-erase-your-hard-drive.html

Parted Magic is mentioned as support Secure Erase for SSDs and HDDs.
See https://partedmagic.com/secure-erase/ and
https://partedmagic.com/nvme-secure-erase/. I remember using Parted
Magic a long time when it was free. Apparently it has gone payware; see
https://partedmagic.com/store/. There are other places to find a secure
erase tool, like https://cmrr.ucsd.edu/resources/secure-erase.html, but
I don't know if it works on all brands of SSDs and HDDs.

Why are you afraid of Dell seeing any data on the returned drive? Just
do a full format (not a quick format) on the drive. Doesn't matter if
partitioning is left behind. If concerned about old sectors having old
data in them that are not currently allocated, use CCleaner or Erase to
wipe free space.

jason_warren@ieee.org <jason_warren@ieee.org> wrote:
> What's the best way to clean everything off a hard drive
> before I return it to Dell?
>
> Will a low-level format do it or is there something better>
> The drive has five partitions.
>
> Thanks!

As others mentioned, it's unclear if this is a system disk - i.e.
includes C: and perhaps other system partitions - or is a non-system
disk.

If the latter, I would use the 'diskpart' utility and do a 'clean all'
command for each partition and then delete all the partitions.

'clean all':

"Specifies that each and every byte\sector on the disk is
set to zero, which completely deletes all data contained
on the disk."

'diskpart' is not for the faint-of-heart, so make sure you know what
you're doing!

Use 'list disk' to list the disks on your system and note - by
checking the Size - which disk is which. Then use 'select disk <N>' to
select the correct disk. Then use 'list partition' to list the
partitions, 'select partition <N>' to select a partition, 'detail
partition' to get info on the partition, 'clean all' to completely erase
the partition and 'delete partition' to delete the partition. Rinse and
repeat for the other partitions.

If this is a system disk, you can do the same from a seperate bootable
medium. For example, I used the 'Rescue media' made by Macrium Reflect.

HTH.

Hello.

"jason_warren@ieee.org" <jason_warren@ieee.org> schrieb

>Will a low-level format do it or is there something better>
>The drive has five partitions.

What is in this time and age a lowlevel format?
Ages ago it was possible for a consumer to low level format old drives
like MFM or so.
Today drives do not allow low level format.

If you want to return a functioning drive to the seller/manufacturerer
and you are afraid they have in this time and age the time and money
to look for your Vacationpictures, then overwirte it.
HDTunepro erase with random numbers, or dariks boot and nuke, or other
tools provide the feature to overwrite the complete disk with random
or special patterns.
Even windows diskpart command 'clean' can overwrite the whole disk
(with zeros).

If you are afraid someone like NSA or so wants to see you
Vacationpictures: some tools provide further methods to overwrite
multiple times with special patterns....

But all of that takes time!
22TB 3.5 inch HDD overwrite one time= 26 to 28 hours.

On 11/27/23 13:50, Shinji Ikari wrote:
> Hello.
>
> "jason_warren@ieee.org" <jason_warren@ieee.org> schrieb
>
>> Will a low-level format do it or is there something better>
>> The drive has five partitions.
>
> What is in this time and age a lowlevel format?
> Ages ago it was possible for a consumer to low level format old drives
> like MFM or so.
> Today drives do not allow low level format.
>
> If you want to return a functioning drive to the seller/manufacturerer
> and you are afraid they have in this time and age the time and money
> to look for your Vacationpictures, then overwirte it.
> HDTunepro erase with random numbers, or dariks boot and nuke, or other
> tools provide the feature to overwrite the complete disk with random
> or special patterns.
> Even windows diskpart command 'clean' can overwrite the whole disk
> (with zeros).
>
> If you are afraid someone like NSA or so wants to see you
> Vacationpictures: some tools provide further methods to overwrite
> multiple times with special patterns....
>
> But all of that takes time!
> 22TB 3.5 inch HDD overwrite one time= 26 to 28 hours.

If you overwrite the entire drive with zeros, everything
on it is toast. There is no way to recover it. And you
do not need to overwrite several times. If that was possible,
there would be so many error writing and reading in normal
operation, the drive would be unusable. There are no shadows
either. That is a myth. If that were even possible, the
drive manufactures would use it to build larger drives.

And keep in mind, if the bad guys are well funded, they'd
just breaking your house and steal your whole computer.

On 11/27/23 10:38, T wrote:
> On 11/27/23 10:12, Carlos E. R. wrote:
>> On the original IBM PC and clones, which used hard disks with stepper
>> motors, the low level format was accessed from the debug command, to
>> run directly a routine in the firmware.
>
> Now that was a trip down memory lane.  :-)

Carlos,

Are you old enough to remember the 10 MB Miniscratch
drives?

Man they were loud when they failed! Fingernails
down a chalk board on steroids.

-T

On 2023-11-28 01:46, T wrote:
> On 11/27/23 10:38, T wrote:
>> On 11/27/23 10:12, Carlos E. R. wrote:
>>> On the original IBM PC and clones, which used hard disks with stepper
>>> motors, the low level format was accessed from the debug command, to
>>> run directly a routine in the firmware.
>>
>> Now that was a trip down memory lane.  :-)
>
> Carlos,
>
> Are you old enough to remember the 10 MB Miniscratch
> drives?
>
> Man they were loud when they failed!  Fingernails
> down a chalk board on steroids.

I'm guessing what you mean, but no, I never met those.

I can tell you another tale.

The computer that failed to boot in winter till it warmed for 15 minutes.

When the hard disk warmed a bit, the head got aligned and could read the
boot sector and files. When cold, it would not boot.

Step motor.

A 32 meg drive, that was my first HD.

--
Cheers,
Carlos E.R.

T <T@invalid.invalid> wrote:

> And keep in mind, if the bad guys are well funded, they'd
> just breaking your house and steal your whole computer.

Nah, they'd just clone the drive and leave. You'd never know they were
there until you received a court appearance order for all those
instructions on how to build WMDs.

On 2023-11-28 00:37, T wrote:
> On 11/27/23 13:50, Shinji Ikari wrote:
>> Hello.
>>
>> "jason_warren@ieee.org" <jason_warren@ieee.org> schrieb
>>
>>> Will a low-level format do it or is there something better>
>>> The drive has five partitions.
>>
>> What is in this time and age a lowlevel format?
>> Ages ago it was possible for a consumer to low level format old drives
>> like MFM or so.
>> Today drives do not allow low level format.
>>
>> If you want to return a functioning drive to the seller/manufacturerer
>> and you are afraid they have in this time and age the time and money
>> to look for your Vacationpictures, then overwirte it.
>> HDTunepro erase with random numbers, or dariks boot and nuke, or other
>> tools provide the feature to overwrite the complete disk with random
>> or special patterns.
>> Even windows diskpart command 'clean' can overwrite the whole disk
>> (with zeros).
>>
>> If you are afraid someone like NSA or so wants to see you
>> Vacationpictures: some tools provide further methods to overwrite
>> multiple times with special patterns....
>>
>> But all of that takes time!
>> 22TB 3.5 inch HDD overwrite  one time= 26 to 28 hours.
>
>
> If you overwrite the entire drive with zeros, everything
> on it is toast.  There is no way to recover it.  And you
> do not need to overwrite several times.  If that was possible,
> there would be so many error writing and reading in normal
> operation, the drive would be unusable.  There are no shadows
> either.  That is a myth. If that were even possible, the
> drive manufactures would use it to build larger drives.

There are suspicions that a lab could read residual magnetism on the
sides of a track, but nobody with that sort of knowledge has clarified
the doubt in any direction. So there was software what would overwrite
them several times with several patterns, just in case it was really
possible.

It makes no sense on memory media like ssd, and it actually damages the
disk. Once is enough.

>
> And keep in mind, if the bad guys are well funded, they'd
> just breaking your house and steal your whole computer.you

Then you would know they did it, and those guys sometimes also want you
not knowing they know.

--
Cheers,
Carlos E.R.

On 11/27/2023 10:45 AM, jason_warren@ieee.org wrote:
> What's the best way to clean everything off a hard drive
> before I return it to Dell?
>
> Will a low-level format do it or is there something better>
> The drive has five partitions.
>
> Thanks!
>

Boot Windows installer DVD.

Or, make the System Repair disc shown here, if you
don't have an Installer DVD handy. This is a 300MB or
so boot.wim which boots and gives access to Command Prompt.

https://i.pcmag.com/imagery/articles/039d02w2s9yfZVJntmbZVW9-51.fit_lim.size_1072x.png

A Macrium Rescue CD also has its own command window and
you could do it from there.

In Troubleshooting, find Command Prompt. AFAIK it runs as Admin.

diskpart

list disk
select disk 0 <=== Make absolutely sure you have the correct disk. NO UNDO.

clean <=== this only removes partition table related stuff.
Very fast. Not forensically clean. Not recommended.

clean all <=== Will write 1TB of zeros to a 1TB drive, end-to-end.
All partition tables destroyed. All partitions destroyed.
Not recoverable, as it is all zeroes.

exit <=== It might take an hour or two, before you can enter this.

You would need a hex editor at this point, to prove it's clean.

While this is tempting as a partial proof, it's also silly.

dd if=/dev/sda bs=221184 | sum # Would return 0000 given a chance.
# If nonzero, there is a screwup.
# But this takes too long to be a useful idea.
Paul

On 11/27/23 17:43, VanguardLH wrote:
> T <T@invalid.invalid> wrote:
>
>> And keep in mind, if the bad guys are well funded, they'd
>> just breaking your house and steal your whole computer.
>
> Nah, they'd just clone the drive and leave. You'd never know they were
> there until you received a court appearance order for all those
> instructions on how to build WMDs.

Takes too much time! Well, unless they wanted to hide
their activity and yo were away on a fishing trip.

On 11/27/23 17:45, Carlos E. R. wrote:
> On 2023-11-28 00:37, T wrote:
>> On 11/27/23 13:50, Shinji Ikari wrote:
>>> Hello.
>>>
>>> "jason_warren@ieee.org" <jason_warren@ieee.org> schrieb
>>>
>>>> Will a low-level format do it or is there something better>
>>>> The drive has five partitions.
>>>
>>> What is in this time and age a lowlevel format?
>>> Ages ago it was possible for a consumer to low level format old drives
>>> like MFM or so.
>>> Today drives do not allow low level format.
>>>
>>> If you want to return a functioning drive to the seller/manufacturerer
>>> and you are afraid they have in this time and age the time and money
>>> to look for your Vacationpictures, then overwirte it.
>>> HDTunepro erase with random numbers, or dariks boot and nuke, or other
>>> tools provide the feature to overwrite the complete disk with random
>>> or special patterns.
>>> Even windows diskpart command 'clean' can overwrite the whole disk
>>> (with zeros).
>>>
>>> If you are afraid someone like NSA or so wants to see you
>>> Vacationpictures: some tools provide further methods to overwrite
>>> multiple times with special patterns....
>>>
>>> But all of that takes time!
>>> 22TB 3.5 inch HDD overwrite  one time= 26 to 28 hours.
>>
>>
>> If you overwrite the entire drive with zeros, everything
>> on it is toast.  There is no way to recover it.  And you
>> do not need to overwrite several times.  If that was possible,
>> there would be so many error writing and reading in normal
>> operation, the drive would be unusable.  There are no shadows
>> either.  That is a myth. If that were even possible, the
>> drive manufactures would use it to build larger drives.
>
> There are suspicions that a lab could read residual magnetism on the
> sides of a track, but nobody with that sort of knowledge has clarified
> the doubt in any direction. So there was software what would overwrite
> them several times with several patterns, just in case it was really
> possible.
>
> It makes no sense on memory media like ssd, and it actually damages the
> disk. Once is enough.
>
>>
>> And keep in mind, if the bad guys are well funded, they'd
>> just breaking your house and steal your whole computer.you
>
> Then you would know they did it, and those guys sometimes also want you
> not knowing they know.
>

I remember strutting like a peacock when we got our first
20 MB hard drive

On 11/27/23 17:38, Carlos E. R. wrote:
>> Carlos,
>>
>> Are you old enough to remember the 10 MB Miniscratch
>> drives?
>>
>> Man they were loud when they failed!  Fingernails
>> down a chalk board on steroids.
>
> I'm guessing what you mean, but no, I never met those.

The dark side of MiniScratch (MiniScribe):

https://www.gillware.com/hard-drive-data-recovery/the-first-hard-drive-to-brick/

On 11/27/2023 2:09 PM, Ed Cryer wrote:
> jason_warren@ieee.org wrote:
>> What's the best way to clean everything off a hard drive
>> before I return it to Dell?
>>
>> Will a low-level format do it or is there something better>
>> The drive has five partitions.
>>
>> Thanks!
>>
>
> Just do a factory reset.
> Settings, System, Recovery, Reset PC, Remove Everything, and make sure you select the Clean the drive option to ensure everything gets deleted.
>
> Oh, and don't listen too closely to the discussions of Win10 chummies. They're far more interested in the internal philosophy of matters concerned than the practical task you've raised.
>
> Ed
>

Sorry Ed, this is still wrong.

You method has merit IFF:

1) You make recovery media, like the DVD set that
PCs used to make. Today, they do not typically
make those DVD sets. An alternative today, is to
make Retail media using the downloaded Microsoft ISO.
This leaves your Dell with something the staff can boot,
to prove no damage to the hardware. But the staff will
still have to do a factory-restore, using their own media.
The staff are not allowed to trust that the customer
did a good job.

2) Once you have the media prepared for a bare metal restore/factory recover,
*physically erase the drive*. This is important. Some
of the other respondents have mentioned some methods to do this.
A method in past years, was DBAN (single pass mode is OK).

3) Now, the disk is clean as a whistle (absolutely nothing on it).
Now do the factory restore, using the optical media. It is
at this point, it is now *truly* in factory state. Any white spaces
on the drive at this point, contain all-zeros, and none of your
passwords or banking info, are on the disk.

It's the white spaces on the drive, after the factory restore,
that your method misses. This is why step (2) is important.

Doing a factory on-disk only (no optical media goes into the tray),
guarantees your personal details are left all over the place. Not good.

There are people who REVEL in recovering left behind customer info.
They don't generally do it for a profit motive, but
they like to make an example out of people who
don't do a good job.

Even on methods that are supposed to work, I've tested
them and GOD DAMN if they still didn't leak. I got around
200 hits in a hex editor, for my test pattern, after
a method was used that was supposed to remove the stuff,
didn't work as well as expected. That's 200 file chunks
that should not have been there.

This is why some of the answers are extreme -- the participants
want to make sure there are no leaks. A Dell or HP returns
specialist, will have materials to redo the disk anyway, so
it's not a problem if it arrives completely blank. It's convenient
if it boots into something, but not absolutely necessary.
Perhaps for factory/warehouse security reasons, they
don't allow any "customer-boots" to occur, and instead
just wipe everything the same way. A customer for example,
could leave a malware on the hard drive.

Paul

On 11/27/2023 4:50 PM, Shinji Ikari wrote:

> Even windows diskpart command 'clean' can overwrite the whole disk
> (with zeros).

The diskpart.exe command is "Clean All" to zero a hard drive. This takes
an hour or two.

There are two commands. "Clean" is one command.
"Clean All" is the other command. "Clean All" is
the one that zeroes a drive from end to end. You can
use the HxD hex editor, open the drive at the physical
layer, spin the scroll bar, and you'll see nothing
but zeros after that command is finished, the "Clean All".

Paul

On 11/27/23 20:40, Paul wrote:
> This is why some of the answers are extreme -- the participants
> want to make sure there are no leaks. A Dell or HP returns
> specialist, will have materials to redo the disk anyway, so
> it's not a problem if it arrives completely blank. It's convenient
> if it boots into something, but not absolutely necessary.
> Perhaps for factory/warehouse security reasons, they
> don't allow any "customer-boots" to occur, and instead
> just wipe everything the same way. A customer for example,
> could leave a malware on the hard drive.

There is a used computer store a few towns away. Customers
that have purchased them, have found (and showed me) all
kinds of sensitive documents on them. And everyone around
has heard the stories, so no one trusts them.

Me wonders ow they got their used computer store Windows
license on the without reformatting them.

On 11/27/2023 8:45 PM, Carlos E. R. wrote:

> There are suspicions that a lab could read residual magnetism on the sides of a track,
> but nobody with that sort of knowledge has clarified the doubt in any direction.
> So there was software what would overwrite them several times with several patterns,
> just in case it was really possible.

Modern drives use vertical recording. If you find
an MFM picture of a vertical recording platter,
it's "black" between tracks. There is no indication
of a fringing field.

In vertical recording, there is a keeper layer below
the plating stack. The recorded bits are vertical ovals.
The keeper layer is the bottom boundary of the magnetic
circuit

On the old drives, recording was longitudinal, on the
surface of the platter. There is still a plating stack
there, but no need for a keeper layer. Longitudinal
recording had more obvious fringes.

Some of the thoughts on the topics from the old
days, are addressed here.

https://en.wikipedia.org/wiki/Gutmann_method

The modern Secure Erase pattern, is single pass.
The drive does not do 35 passes for Secure Erase.
The time to Secure Erase a drive, is pretty well
identical to the time to write from end to end.
Secure Erase or Enhanced Secure Erase, attempt
to "write everything if they can". (The sparing
sector area is erased too.) If you wanted to emulate
35-passes, you could always execute Secure Erase
35 times, but that would be silly.

Governments with "Top Secret" class hard drives,
do not take chances, and they use a mechanical
shredder to ensure all the magnetic media is
bent to some degree. An MFM is only good on
perfectly flat media (media which is as flat as
it was when it was written). The forces are measured
on the Z-axis, at the nanometer level. You would
think any curvature to a sample, would cause a problem.
A typical X-Y scanning area for an MFM, is 100u x 100u
(microns). There are something like 1200 MFMs, in places
like university labs. The grad students, hardly ever
seem to be sticking disk platters in their machines :-)
For one thing, there isn't a positioner with a 3.5"
displacement, inside the machine. If you wanted to
scan a platter, you'd have to add some kit to the machine.
The tempco of the platter, could easily degrade
attempts to push a platter around to a 3.5" limit.

Paul