There is for all intents no such animal as a "single density drive". Some can, some can't. The 765 FDC core knows both and and *should* but with the bare 765 chip that requires a bit of external logic around clocks and data seperators. For the later integrated 765 cores like the 37C65 it's more a programming issue. One caveat on that is the slowest rate (125kb/s 5.25 SD) is often not available however if you have a standalone XT or AT class clard the typical 16mhz clock on the board can be replaced with a 8mhz (move all the data rates down by 50%). I have such a hacked board as it was an older one using a 765A and came with a rare thing, a schematic. However bios support for such a thing is NOT there and you have to do all the work and even for an unhacked board the bios may be less than helpful. Allison