Reply 20 of 24, by mkarcher
Matth79 wrote on 2022-04-13, 23:13:A permanent magnet strong enough to wipe the disk, would leave it harder for the drive to write as the disk would be fully magnetised
A working floppy drive should generally create a field strong enough to correctly write to the medium, as long as it is a supported medium. DD drives may fail to handle HD media, and even more so when the disk is fully magnetized. A floppy drive write head is built in a way that it writes data to the center of the track and erases data (using AC magnetization) at both edges of the track, to prevent old data disturbing the reading of the new data, in case the new track doesn't perfectly align with the old track.
This means that low-level formatting a floppy disk just once in a properly working drive is enough to remove any disturbing old magnetization that could cause problems when you read the floppy afterwards in the same drive (or same kind of drive, as long as both drives are aligned in the same way). The only moment the method of writing the center and erasing the edges fails is when the writing drive and the reading drive do not agree on track width, such that the reading drive reads tracks wide enough that it reads outside of the area erased by the writing drive. This can happen when you write on an 80-track drive and plan to read with a 40-track drive. In case of PC-compatible drives, this only applies to writing using a 1.2MB drive and reading using a 360KB drive. Bulk erasing (preferrably using proper AC degaussing) might actually help in this isolated case. In all other cases, degaussing a floppy won't improve results over just trying to reformat it without degaussing it in-between.
The pure act of reformatting a floppy can in fact "cure" bad sectors, if the problem is caused by the surface being dirty. The dirt might be wiped by the tissue inside the medium, maybe only after being gently "scratched" by the head. Furthermore, as the heads push from both sides onto the medium, just repeatedly formatting a disk might straighten out dents in the medium. In both cases, the surface improves due to the mechanical treatment during formatting, not due to the magentization applied during formatting, so degaussing can't assist that process.