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Unformatting floppies with bad sectors with a tape eraser

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khaz

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I was given a bulk tape eraser, that I tested on a bad floppy I had. It had bad sectors that were consistently spotted during a format (with the drive head grinding a bit then marking the sector bad). After the magnetic erasure, the drive was able to format it completely, without ever grinding on a problematic sector. What is the current status of this floppy? Will it fail as soon as a write is attempted, or is it good but the sectors are still weak and will fail again at some point? Or is the floppy entirely cured and good as new?
 
It is theoretically possible that a spot on a floppy disk may have become saturated with a magnetic field larger than what the drive head can overwrite (that is actually what prevents formatted 1.2mb disks from being formatted as 360k)

But it is much more likely that the act of running the disk through the drive several times removed some dirt or debris.

If it was just junk on the disk then it very well may continue to work fine. However if it was a minor surface defect it it may crop up when you try to write or read from the disk in the future.

Personally, what I would do when in doubt like this, is copy a zip file or something with random-ish data to the disk and then run Norton Disk Test a few times.

If you have one, the SuperCard Pro has a low-level media test tool. (Although that may only work with low density media).
 
I've been able to resurrect 3.5" floppies with bad sectors by plonking a big speaker magnet on the disk and waving it around a few times. It's hit or miss on how long it lasts though, as sometimes it's media degradation, while others it's just the magnetic signature on the disk being in some invalid state.
 
The disc behaves fine for now. I'll do more of them.

What causes these bad sectors any way?
 
Also it depends on what you meant when you said you "formatted" the disk. A DOS or Windows "quick format" doesn't actually format the disk and won't fix bad sectors. A "full format" should have and if it didn't it would be interesting to work out why.
 
I only did
> format a:
under DOS 6.22

afaik sector status is reinitialised at the beginning of the formatting process. If a sector is found impossible to format, it is marked as bad in the FAT and can't be used to store data.

This specific floppy had been formatted several times before getting the electromagnetic treatment, and each time reported the same amount of bad sectors.
 
The reason why it works is because a floppy drive can only erase the part of the disk that is directly underneath the heads, while the bulk tape eraser erases the entire magnetic surface of the disk. It rearranges the magnetic particles in random order across the disk, which can help mitigate magnetic weak spots and strong spots that may have formed on the disk.
 
khaz said:
This specific floppy had been formatted several times before getting the electromagnetic treatment, and each time reported the same amount of bad sectors.
Click to expand...

Then it may not have been trying to actually format the bad sectors. DOS 6.22 takes some shortcuts without additional options; a format is really a "verify readable". To truly force a format, use /u as a command-line switch.
 
When I'm reclaiming floppies, I use a copy-with-format-and-verify utility, so I've verified that the format is writable as well.

I use a 6-drive system with 3 controllers, so I can work on 3 floppies simultaneously, with three more ready for formatting or just completed.
 
Trixter said:
Then it may not have been trying to actually format the bad sectors. DOS 6.22 takes some shortcuts without additional options; a format is really a "verify readable". To truly force a format, use /u as a command-line switch.
Click to expand...

I didn't know that, I'll have to try it with the next floppy giving me trouble.
 
In newer versions of DOS, if you reformat a disk that has already been formatted without specifying the /U (unconditional) switch, it will say "Verifying 1.44MB..." (or other capacity) instead of "Formatting 1.44MB..." and will skip over tracks that had been marked as bad during the previous format.

Note that for some reason in Windows 95 and above, the /U switch is undocumented, but it still works.
 
Chuck(G) said:
When I'm reclaiming floppies, I use a copy-with-format-and-verify utility, so I've verified that the format is writable as well.

I use a 6-drive system with 3 controllers, so I can work on 3 floppies simultaneously, with three more ready for formatting or just completed.
Click to expand...

I'm curious to see what this looks like. Can I trouble you for a few pics?
 
I snapped a couple of quick ones, sorry for the blur--this was hand-held. I don't know how to rotate the photo of the box so it appears right-side up.

The first is a photo of the screen I just set up; the second is the working part of the tower with the drives--currently 5 3.5" and 1 5.25". Motherboard is a 386 of uncertain vintage; hard disk is a CDC Wren SCSI connected to an Ultrastor 14N controller, which also supplies the first FDC. The other two FDCs are Micro Solutions Compaticard IV units, each on a separate port, DMA and IRQ.

All FDCs, including the Ultrastor can support 4 floppy drives, so I could have set up a parallel external box with 6 more drives in it.
 

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wow. That's a neat card and setup. I can see myself building such a system, but with only 4 drives. thank you!

but back on topic, I have personally used a "bulk tape eraser" to wipe a disk. It does seem to completely obliterate any data on the disk, and it was useful when I was formatting 1.4mb disks to 720k as a stopgap measure when I didn't have any dsdd around. I wouldn't use it for anything long term though.
 
Stone said:
BTW, you should be using IrfanView (by now). :)
Click to expand...

Amen to that.

I reclaim floppies of uncertain heritage by first wiping both sides with a neodymium-alloy magnet to erase any traces of former format markers and then using an unconditional format to reformat them. I pay attention to the sound the drive makes and if there is any interruption to the steady stepping rhythm - indicating a retrace over a difficult sector - the disk goes in the wastebasket. Not worth the risk, in my opinion.

-CH-
 
I have found that over time many floppies with a few bad sectors can maintain their integrity once the bad sectors have been removed from service. In most cases it's not contagious like a disease but rather just an isolated flaw.

Don't forget -- all hard disks are born with surface errors that have been removed from service. :)

I also like to use a neodymium magnet. And since every dead hard drive seems to have a free one inside... :)
 
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