Had to have the logic board in my Mac Pro replaced due to faulty audio input jack. Time machine backs up to an external drive (DROBO array). When I brought the machine back home, time machine does not recognize the old backup (last backup never) and doesn't have room to backup a new system because the old TM files take up most of the drivespace. Is there a way to "force" time machine to accept an old TM backup file? I don't want to delete everything and start again. I assume this is because the logic board (motherboard) was replaced. Note that the system drive and all internal hard drives were not modified, and the OS was not reinstalled or anything like that. One other item: the tech forgot to run some software at apple and my machine currently has no serial number (about this mac...) assigned. They are sending out a tech to fix this - does time machine back up based on serial number? If so, maybe that will fix the problem. thanks for any help!