As for the WD model mentioned in my last post, colleagues who use it for critical business backups (on multiple Macs, using Time Machine) tell me that it does the job right out of the box (ie, they've never had to reformat the drive prior to use).
The reason for that, I just noticed (Pogue, p 262), is that "Time Machine automatically reformats [drives] to the Mac OS Extended (Journaled) scheme, which it requires."
Reformatting of the entire drive only takes place if/when the external is physically mounted via a
direct cable connection. Not sure, but if your muliple-Mac-using colleagues are backing up to the same drive, chances are they're using Wi-Fi rather than running around the office shuttling the backup disk back and forth.
In the case of a NAS (network attached storage), Time Machine doesn't reformat the actual remote disk itself. Rather, it creates a disk "image" on that filesystem, mounts that file as if it were an actual disk, and reformats just that imaginary disk. It's kinda abstract, but less work for the user.
E.g., i'm not sure what format the disk is inside a Time Capsule, but i do know from several attempts that it doesn't support hard links (Unix-style). Therefore it isn't UFS or HFS+ format. But no matter, Time Machine can back up to it anyway, since it uses that disk image method just described. [fwiw, Time Machine relies heavily on Unix hard links.]
