>I'm not really familiar with Mr. Atari's "My IDE Interface", but I think it can >go well over 16MB.
As for info about Atari diskdrives:
http://www.faqs.org/faqs/atari-8-bit/faq/section-20.html
As for the 16MB limit, this is a DOS limit. In those days they never thought about such large volumes, and to maintain compatibility all DOS' do support ''only'' up to 16MB.
Back then, a special version of MyDOS claimed to have 48MB as the limit, but it was never released to the public.
A new DOS is in the make (albeit a while since I received the last beta-update) which is compatible with the Sparta-Dos diskstructure but with virtually no limits, however the limit will now be fixed to 64MB max.
Atari ST diskdrives are not compatible in any way to a standard Atari XL and XE computer. An ST drive is just a bare disk-mechanic (like Amiga/PC) while the floppycontroller is built into the computer, where the XL/XE drives are ''intelligent'' devices and have their own processors.
Several third-party upgrades for the XF551 do exist to attach an extra 720KB 3.5'' mechanic to the diskdrive in addition to the original 360KB 5,25'' one, but one can simply exchange the standard 40 tracks EPROM of an XF551 for an 80 tracks one to support double-sided 720KB disks. When using an EPROM which is twice as big you can easily switch between the ''old'' 40 tracks code and the new 80 tracks.