vico wrote on 2025-03-18, 17:08:

I use magnet from failed HDD - fits perfectly for 3.5" diskettes. Usually I touch the magnet to one side of the diskettes's plastic wrapper/envelope/case and manually spin the magnetic disk a couple of turns. Then on the other side. And finally format with NFORMAT.EXE usually results in healthy diskette with no bad sectors. Sometimes I have to repeat this procedure more then once.

I've still got a couple ancient "bulk tape erasers" - there are strong electromanets operating at 60hz line power.
How they work is thay scramble the magnetic domains within the media, and you move them away while still powered.
This causes the field setting a magnetic domain to grow weak, so less and less domains "line up" at the end.
Effectively removing magnetic traces.

A permanent magnet will not alternate the domains, and will leave them strongly oriented in one direction
- this may be more than the floppy head can completely re-align, resulting in a less reliable disk.

If you don't have such an eraser, use an electromagnet driven by a suitable transformer, and keep moving it
around the disk as you pull it away.

DaveDDS wrote on 2025-03-18, 18:38:

A permanent magnet will not alternate the domains, and will leave them strongly oriented in one direction
- this may be more than the floppy head can completely re-align, resulting in a less reliable disk.

Those neodymium magnets removed from a HDD actuator are not so strong (especially after some heat is used to melt the glue while removing them) as you think, otherwise with time they would corrupt the magnetic records on the HDD's platters.

Example YT video: https://www.youtube.com/watch?v=9LdcyuuBfME&t=443s

P.S. Actually I was searching for this BuB's video: https://www.youtube.com/watch?v=lDnvF9sXgE0

Got a ZIP 250Mb disk with a single bad sector located exactly at space where FAT is intended to be.
If I remagnetize the disk as described earlier, I need to low-format the disk (all standard methods do a quick-format). Are there any way to revive this disk back to life?

analog_programmer wrote on 2025-03-18, 19:33:

Those neodymium magnets removed from a HDD actuator are not so strong (especially after some heat is used to melt the glue while removing them) as you think, otherwise with time they would corrupt the magnetic records on the HDD's platters.

Actually... they might be stronger than you think.... In HDD they are usually within some well magnetically
shielded component, and are purposely positioned away from the "spinning rust" - the strength of a magnet
decreases rapidly with distance.. (I expect it's something like square of the distance).

When you put them against the disk protective case/wrapper, you are getting them about as close as you
can without actually touching.

But the "big" reason to use reducing alternating magnetic fields when erasing media is to reduce the residual magnetic
domains as close to zero as possible - any permanent magnet will leave residual domains in other magnetic material
when moved in close proximity.

DaveDDS wrote on 2025-03-19, 16:41:

But the "big" reason to use reducing alternating magnetic fields when erasing media is to reduce the residual magnetic
domains as close to zero as possible - any permanent magnet will leave residual domains in other magnetic material
when moved in close proximity.

And what can guarantee that residual magnetism in some of the magnetic domains will get close to zero when using some "very special" electromagntic eraser, if there is a maximum (peak) in created alternating magnetic field?

The method with those small neodymium magnets from modern HDD's actuators just works fine for wiping-out the diskettes including their magnetically (logical) bad sectors.

analog_programmer wrote on 2025-03-19, 17:13:

And what can guarantee that residual magnetism in some of the magnetic domains will get close to zero when using some "very special" electromagntic eraser, if there is a maximum (peak) in created alternating magnetic field?

There is of course no guarantees, but you can take steps to make them as close to effectively zero as possible.

This the the reason you don't just "turn off" the eraser in close proximity of the media.

As I mentioned earlier, the magnetic field strength grows rapidly weaker with the distance from a magnet.
and you are correct in that it you "switched off" at the peak (strong field) the effect would be no different
that a permanent magnet.

When erasing media, the alternating magnetic field will pull the domains alternately back and forth..
As you pull the eraser away, this effect will grow less and less, with fewer domains "switching" and as hard
than when the field was strong. As you move away, the field will grow closer to zero and the domains
orientation will closer approach "random".

A permanent magnet only aligns the domains in one direction..
Pulling a permanent magnet away slowly does not cause them to re-align as is the effect from a alternating
field.

DaveDDS wrote on 2025-03-19, 18:33:

A permanent magnet only aligns the domains in one direction..
Pulling a permanent magnet away slowly does not cause them to re-align as is the effect from a alternating
field.

Mind that you don't hold the magnet with only one of its poles pointing to the diskette and by moving the magnet over in random directions you're actually creating non-constant magnetic field.

Ok, the special dedicated eraser tool does work for sure, but it's proven not just by my own practice, that there's nothing wrong with using a small neodymium magnet instead.

Another example: In the late '90s there were a very cheap chinese double cassette ghetto blasters witch erasing "head" was actually a permanent magnet (which makes contact directly with the tape when recording). Those erasing "heads" create the specific background noise which is explained by the of the magnetic domains in one direction orientation, but it doesn't make the tape unerasable, unrecordable or with remaining excessive "hiss" after it was used again for record in cassette deck with normal electro-magnetic erasing head.

analog_programmer wrote on 2025-03-19, 19:49:

Mind that you don't hold the magnet with only one of its poles pointing to the diskette and by moving the magnet over in random directions you're actually creating non-constant magnetic field.

Yeah, but I've not actually SEEN a permanent magnet with both poles in the same place.... and most are long enough (couple mm is lots) that the "1 direction" effect will
still be present, so unless you swap poles (over the same position) faster than you are moving the magnet away... you're still going to
have more residual effect. An electromagnet on AC *does* effectively have both poles in the same and can swap them much faster than
you typically "pull it away".

Ok, the special dedicated eraser tool does work for sure, but it's proven not just by my own practice, that there's nothing wrong with using a small neodymium magnet instead.

In 99.9% of cases it won't matter - the very slight unreliability from having existing magnetic domains on the media won't affect it's ability to store/retrieve digital streams *in almost all cases*
- it will however fairly strongly affect the ability of "bad guys" to recover your "erased" data.

- having said that, it's been ()x() years since I worked in an environment where we actually worried about such things -
I only use the erasers now because I happen to have them - If I didn't, I'd probably use an electromagnet on a transformer if I wanted to
"be sure", and likely just a PM if I'm trying to see if I can recover bad media (weird ... I actually remembered what this topic was about)

Another example: In the late '90s there were a very cheap chinese double cassette ghetto blasters witch erasing "head" was actually a permanent magnet (which makes contact directly with the tape when recording). Those erasing "heads" create the specific background noise which is explained by the of the magnetic domains in one direction orientation, but it doesn't make the tape unerasable, unrecordable or with remaining excessive "hiss" after it was used again for record in cassette deck with normal electro-magnetic erasing head.

It wouldn't make media unusable - just not quite as reliable as it could be in some cases (which can be fixed with proper full erase)
- I do think a key phrase here is "very cheap Chinese" ... not always the best reference on how to properly do things.

** I personally suffer from a fairly severe curse - I *have* to know how things work! I can't use "stuff" without knowing exactly what's
going on inside - same for for processes (like magnetic recording) - I do tend to get a bit anal about "the right way to do" things ...

analog_programmer wrote on 2025-03-18, 17:21:

vico wrote on 2025-03-18, 17:08:

Does this method work to fix the error: "Track 0 bad disk unusable"?

It should work, if the floppy disk is not physically damaged.

I was able to reduce the bad sectors on one of my drives to 1024 bytes, and I was able to format another drive without bad sectors. However, I had no luck with my two "Track 0 bad disk unusable" floppy disks; they remain unusable.

Thanks for sharing this method.

vico wrote on 2025-03-20, 04:34:

However, I had no luck with my two "Track 0 bad disk unusable" floppy disks; they remain unusable.

Did you use the magnet on them before formatting?

Here is a video with track 0 problem - the guy uses some eraser tool on the diskette, but he also says that any magnet will do the same job: https://www.youtube.com/watch?v=49Pcmgh7tPc

analog_programmer wrote on 2025-03-20, 11:30:

Did you use the magnet on them before formatting?

Here is a video with track 0 problem - the guy uses some eraser tool on the diskette, but he also says that any magnet will do the same job: https://www.youtube.com/watch?v=49Pcmgh7tPc

Yes, several times, with magnets I got from an old ruined IDE hard drive.

vico wrote on 2025-03-20, 14:43:

Yes, several times, with magnets I got from an old ruined IDE hard drive.

So, probably there's some damage on the surface of the magnetic disk in the area of track 0 which can't be fixed.

I keep one problematic diskette with slight scratch near the middle tracks of the disk. Its track 0 is healthy, so it's always formatted with couple of bad sectors even after magnet treatment. Still usable tho.

analog_programmer wrote on 2025-03-20, 15:01:

So, probably there's some damage on the surface of the magnetic disk in the area of track 0 which can't be fixed.

Worth doing a close visual inspection - although it's possible to have enough physical damage to make a track unusable,
it's fairly rare - usually you can see physical damage as a more pronounced score mark or small chips/scrape in the media.