hi .it supposed to be 720k..
try mydos 4.5 or spartados 32g for a 3.5 dubble sided dubble density. disks.
most outher dos will do 360k (on a 720 disk.
i just put a stanterd cable with a 5 1/4 plug and a 3.5 in plug, and set the drives as 0 now i have 2 on the same xf551.
i used drive from a old 520st external with power supply.