>Can a 250meg zip disk be used in the 100meg drive?
The medium is capable of being formatted to 100MB; if left in it's original 250MB format, it will not be accessible at all on the 100MB drive.
Think of this like you would an older floppy drive system. Let's limit this to the 5 1/4" medium, in double-sided form. There are essentially two formats available - the standard DS/DD 360K floppy, and the AT-compatible 1.2MB format.
There are two essential differences; the 1.2MB format has twice the number of tracks per inch than the 360K floppy; it fits 80 tracks of data in the same space as the 360K floppy uses to hold 40 tracks of data. The 1.2MB format is capable of resolving lines twice as finely as the 360K drive; it's a matter of the precision of the stepping motor, and the physical width of a discrete magnetic dot on the diskette.
The 1.2MB floppy also holds more information per track; this is because of the precision of the timing and the physical length of one discrete magnetic dot on a track.
The 1.2MB floppy hold twice the number of tracks, and twice the number of bits per track, resulting in 4x the storage. Sounds about right.
You can either use one 'dot' to represent a bit, or four 'dots' (2x2), giving the same resolution as the 360K floppy. The exact mechanical requirements are a bit more complex, but both formats use the same type of NRZI magnetic encoding to hold the data; the issue is just how finely it can distinguish between areas of the disk.
A 1.2MB floppy uses a higher quality recording medium than the 360K medium; it is capable of resolving a bit of data on a finer level, and there's less bleed-through of magnetic bit values. You can format it either to 360K format - a relatively 'wide' and 'long' spacing of bit values on a track, or to the 'thinner', 'shorter' bit size of the 1.2MB format. There are alignment issues that can arise from formatting a 1.2MB medium floppy on a 1.2MB capable drive to 360K, because it writes a narrower stripe, which can be problematic as far as relative alignment.
The 250MB Zip media are capable of accepting a format reliably as either a 100MB disk or as a 250MB disk; the media are capable of equivalent resolution. Formatting a 250MB Zip to 100MB on a 100MB drive will result in a 100MB disk; the issue of formatting the 250MB media to 100MB on a 250MB drive is a bit more complex, because of differences in the magnetic encoding used for 250MB format vs 100MB format. Basically, the 250MB media are capable of accepting the format reliably either way, the same cannot be said of the 100MB medium.
A 100MB Zip drive cannot read media formatted to 250MB at all - it's the same as trying to read a 1.2MB floppy in a 360K drive. It just gets confused.
