If System software prior to Version 6.0 is used with the FDHD, 1.4 MB disks can to be formatted as Macintosh 400K or 800K disks. However, doing so can result in loss of data.
Because the disk surfaces are physically different, using a high-density disk formatted as 400K or 800K can result in data loss.
Using high density disks in a 400K or 800K drive can also result in data loss.