Strange Time Machine Behavior...

[clay]

So I recently upgraded a pair of RAIDed (striped for speed) 1.5tb drives in my Mac Pro to a pair of RAIDed (striped for speed) 2tb drives. Did all the data migration, backups, and all is working fine. But...every time the machine re-boots, one of the 2tb drives is not recognized by the computer. Doesn't show up in Disk Utility or Apple System Profiler. I shut down the machine, slide out the drive caddy, re-insert, reboot, and it shows up and works normally until the next shutdown/reboot instance. Tried different bays in the Mac Pro, same issue.

So...I'm guessing it's an issue with the drive, so I'll RMA it. But...this did cause interesting Time Machine behavior...

When I would restart after reseating the drive, Time Machine would reset itself to the point that it would start backing up the entire dataset (4tb RAID) EVERY TIME. So the same 2.94 TB of data was getting backed up every day as though it were the first time it had ever been backed up. Not sure exactly why it was doing this, because the data on the bad drive hadn't changed, but that's what it's doing.

I'm going to RMA the drive and recreate my striped RAID using some spare drives, and we'll see if this type of issue keeps popping up.

[deckeda]

Time Machine normally treats backup drives as the datasets themselves, which is why when you swap out a drive with a new one it wants to start all over. You're seeing a similar symptom with the flaky RAID drive/controller/enclosure/whatever.

One way around the Time Machine issue to divorce the dataset from the physical drive. The way you do that is to create a sparse disk yourself that sits on the drive and then configure TM to use that sparse disk image.

Doing so will also allow you to copy the sparse disk image at any time onto a different hard disk of your choice and TM won't complain, won't force you to start all over with backups etc. In other words it also permits backing up the TM backup easily.