Floppy Disk Halp — #1

Post #1 by MrMacPlus

Source Forum	68kMLA
Category	Hardware
Post Date	Thu, 21 Aug 2008 - 07:52
Original URL	https://68kmla.org/bb/threads/floppy-disk-halp.14560/

Post

I read that demagnetising a 1440k floppy will essentially make it more compatible with 800k drives.

To clear up the persistent confusion and superstition about 800K vs. 1.44MB media, here's the correct story: There is about a ten percent difference in the magnetization thresholds (called "coercivity") for the two media, with the 800K stuff having the lower value. If you want to get technical, 800K media have a nominal coercivity of 650 oersteds, versus 720 - 730 oersteds for 1.4MB media. So, 800K drives may find it difficult to write on 1.44MB media. However, ten percent is not a large difference, and in fact, is about the same as normal variations within a batch from a given manufacturing run. Plus, coercivity varies with temperature, too. So, the two media are not as wholly incompatible as lore has it.
However, if a 1.4MB disk has ever been written on by a 1.4MB drive (and this includes formatting), an 800K drive's weaker write fields may not be strong enough to reliably over-write the existing data, and you'll have flaky behavior (particularly if you're unlucky enough to have a drive with write currents at the low end of the spec, trying to write on a floppy with coercivity at the high end of spec). But if the floppy is virgin, you'll rarely see any problems at all. You can "re-virginize" floppies if you have a good demagnetizer handy.

I read that from the pickle's low end mac faq. I was able to scramble a floppy with a good old supermagnet taken from a dead HD, but I don't know if that is suffecient, as that is "magnetising" not demagnetising. Also, before I get "yelled" at, I don't have access to 720/800k disks, except for 2 that I found, it obviously would be a better value to use the 20+ HD floppies if I could, instead of buying more expensive and harder to find 720/800k disks. I don't have a demagnetizer around and calling the obvious places in town didn't give results either. One method I also tried was to put the floppy in front of a monitor (and a tv later) and turn it off, then on, thus degaussing the CRT, no result :?: .