Depends on the drive. A late model 800K drive can format disks as MFS 400K or 800K and read and write with no problems. The OS will format 800K under 512K, but won't work under a 128K as it does not have enough memory. The HD20 INIT can also be used to substitute for the 128K ROMs ... Only you cannot boot from an 800K disk ... The early 800K drives can only be accessed using the HD20 INIT.