[dnanian]

I think I suggested booting from your OSX DVD, not the external drive -- unfortunately, neither seems to have worked.

Note that if your drive is named differently than the source, it's quite possible that aliases are resolving to the source, rather than the backup.

So, try booting back to the original drive, then name the backup the same. Then, boot from it and see if it'll unmount properly.

(Very strange you got Kernel Panics simply booting from your OSX DVD. That really implies there's something quite wrong with your system, or you did it with a large number of misbehaving peripherals attached...)