# Algorithm to properly format 360k 5.25″ diskette in a 1.2M drive?

This is a low-level tech question, as I am developing a driver for an FDC at its command-set level.

Having reviewed a number of sources, I cannot find the algorithm to format a 360K diskette using a newer 1.2M drive. Lots talk about the differences in track size, magnetic material, how to degauss the media. But none lays out the algorithm in where heads must be positioned and in what order.

The goal is having a diskette formatted using a 1.2M drive to be portable to a 360K drive as much as possible. I do not have a 360K drive, thus I cannot perform trial-and-error to empirically find out the best way of dealing with formatting.

I had formatted two 360K diskettes using a Windows XP machine recently, and noticed a peculiar behavior: heads start at track 0, to somewhere at the ½ of the surface towards the center in double-track step, then heads go back about 20 tracks, and perform formatting in single track step up to the position where they retracted, and then continue as usual in double track step. This happened on two diskettes, thus it is not caused by the media defect.

There must be some explanation for this behavior, and formatting a 360K disk in a 1.2M drive may not be that straightforward as going from outer to inner in double-track step and writing (formatting) the tracks. What can, and must, I do with the floppy disk controller to format a 360K diskette?

• If your disks have previously been written in a genuine 360K drive, this is unlikely to be successful, because the width of the track written by a 1.2M drive head is about half the width of a 360K head track, so it will never completely erase the original data. The reliable solution is to bulk erase the floppy disks, but I guess you don't have a bulk eraser available. A 360K drive will (usually) read the narrow tracks successfully, so long as there is no "magnetic noise" either side of the narrower strip of data. Apr 25 '20 at 21:34
• "and noticed the distinct behavior: ..." I've never observed this, and I've been using 360k diskettes on MS-DOS. But then, I've never formatted a 360k disk on XP. I don't see any reason why 360k couldn't be formatted in the usual way (track by track). Apr 25 '20 at 22:25
• @Anonymous Sorry, but the only "practical" advice I can offer is this won't work. Unless "as successful as possible" means "100% successful every time," the result is worthless in practice. Apr 26 '20 at 0:38
• Congrats, you found a 30 year old bug in the 360kB on 1.2MB drive formatting method. Apr 26 '20 at 1:14
• I would do a double check with an older Linux distribution first. Apr 26 '20 at 1:16