I use a 500 GB FireWire drive to share files between my two computers (iMac and MacBook Pro) in order to get better performance out of programs like Logic, and not have to worry about syncing my files. Obviously, due to the portable nature of all my data, it is IMPERATIVE that I have a backup.
I have a 2TB Lacie drive plugged into my imac which backs up the iMac and the External dive when it's available, the MacBook Pro is setup such that it backs up to the same drive over the network when it is available. The MBP does NOT backup the external drive when it's connected, it only backs up from the imac. I have several other drives daisy chained on the imac of crap files that don't matter, and they don't back up at all (the external is added to the end of this chain, and the Time machine disk is first, though I can't imagine how this could have any effect.)
Anyways, every time I work on the MBP and then re-plug the drive into the iMac to continue work and do a backup, the time machine starts from scratch and backs up the entire drive.
I thought that my backup may have been corrupt, so I wiped the time machine disk and started fresh, but it's still doing it. This was not happening a few months ago, so I'm not sure what's changed.
Sorry I was unclear. The MBP is fine. The external does the entire backup every time.