> 
> Are tape-changer DDS/DLT/etc tapes sitting in a multi-tape cassette
> much more "offline" then a spun-down, unmounted IDE drive that was
> used as the target for a backup tar once a day?

what i've said is true to tape changers too unless you use this changer
just to have larger capacity per one manual intervention. but all tapes
could be changed/removed at least once a week. in case of worst disaster
- imagine someone cracked into your system and did mt erase on every tape
and then dd if=/dev/zero ... on every disk.
but noone can remotely erase tape or disk that is offline!
 
> I haven't yet gotten up the nerve to try one of those removable IDE
> frames that has a power switch for the IDE drive in the locking
> mechanism.  Unlocking the frame retention slider powers down the IDE
> drive's +5v and +12v.  The company implies that one can swap a drive

i have such frame. and it was working on linux well (with SCSI mounted
root and ide driver as module)