With the motherboard I am using (Supermicro H8SMI-2), the LS120 is treated similar to a CD drive. You can put it in the boot order. Booting from it is like booting from a CD-ROM with floppy emulation. When you do this, the drive appears as A: and the actually floppy drive at A is not available.
When booting DOS from a hard drive, the only way to make the LS120 drive A is to disable both real floppy drives. If you disable just one, the real floppy drive is always A and the LS120 is B. If you have two floppy drives enabled, they are A and B and you need a driver to make the LS120 accessible.
In Windows XP, no driver is required, but the LS120 is never A or B (D by default if you have one hard drive).