The size issue with 48 tpi and 96 tpi r/w heads
Handling recording problems
Having 1.2M drives read or write to a 360K disk is a simple task. Just place a previously formatted 360K disk in the drive and use it normally. In other words, pretend that the drive is a 360K drive. Nothing special must be done. You can either read or write on the disk with absolutely no problems - yet.
You will have a problem if you decide to return the disk to a 360K drive and attempt to read it. Remember that the recorded track width of the 1.2M drive is half the track width of the 360K drive; therefore, if are any tracks have been previously recorded by an actual 360K drive, the tracks are twice as wide as the tracks recorded by the 1.2M drive. If you write to the disk with the 1.2M drive, you cannot overwrite the entire track width—only the center portion of it. When you return this disk to a 360K drive, the wider head system in the 360K drive then sees two signals on any overwritten tracks, and the new data is nestled within the image of the old data that could not be completely covered by the 1.2M drive. An immediate Abort, Retry, Ignore error from DOS usually is displayed for any updated portions of the disk.
To solve this problem easily, if you want to record data in an AT 1.2M drive and later read it properly in a 360K drive, make sure that you use brand-new disks for recording in the 1.2M drive. Because a new disk has no magnetic information on it, the smaller recorded track width can be written on the 1.2M drive and read properly in the 360K drive: The more narrow track is written in "clean space." The 360K drive, therefore, no longer is confused by any "ghost images" of previously recorded wider tracks. Other than starting with a brand-new disk, your only other option is to use a disk erased by a bulk eraser. You cannot erase a disk by reformatting it if has been in use. Formatting records actual data on the disk, and causes the track-width problem. The new or bulk-erased disk in fact must be formatted by the 1.2M drive for this procedure to work again. Remember the simple rule: Any track recorded by a 360K drive cannot be overwritten by a 1.2M drive, even in the 360K format.
Scott Mueller, Upgrading and Repairing PCs, 3rd Edition, page 580/81
The following figure shows the r/w head of the TEAC FD-55E on the right. Since this drive uses 80 tracks (96 tpi), its track width is basically comparable to a 1.2 MB drive.
The recorded track width of a 96 tpi drive (right) is half the track width of a 48 tpi drive (left).