iMac Intel 27" EMC 2309 and 2374 Teardown
Quote from TonyWong:
You would have to downgrade the internal Hard Disk to less than the size of the optical bay HD or use the optical bay HD as the boot drive for Time Machine backups.
The Time Machine backup disk must always be larger than the disk it backs up from, AFAIR.
Nope, you can choose what you backup, and if it fits, it'll do it.