The Atari XF551 was Atari's last disk drive manufactured for the Atari 400/800, XL/XE line of Personal Computers.    The drive was compatible with all previous Atari formats back to the original Atari 810 disk drive 88K single sided/single density format.    The drive could be set to drive #'s 1-4 by configuration the dip switchs on the back of the unit.    The drive was shipped with Atari DOS 2.5 which became of the defacto standard DOS' used by the majority of the Atari community.   However Atari later on would release Atari DOS XE which truly unleashed the Atari XF551 capabilities, speed and full storage.    Atari DOS XE allowed for subdirectories, scripting and many other powerful features only found in such 3rd Party products as ICD SpartaDOS and MS-DOS on the PC Platforms.