Time Machine Disc Unmounts Itself

Question

My Time Machine disc (500 GB FireWire 400) unmounts by itself after 2 to 3 hours. Initially I thought it was the drive. It was always up long enough to get at least one good backup a day so I wasn't too worried about losing much data. Recently, I moved a lot of files (couple hundred GB) and now it can't finish Preparing to Backup before the disc unmounts.
I bought a new disc (2TB FireWire 400) and it is doing the same thing-unmounting after a couple of hours.
However the first disc is now staying mounted (1-1/2 days) with no problems. Obviously it is only the disk defined as the Time Machine disc that unmounts by itself, therefore, it would seem to be a Time Machine problem.
Does anybody know what is going on and, more importantly, how to work around this problem?

Answer 1

Yes, they all drives use Apple Partition Map. I, unfortunately do not have an Intel Mac yet (tears running down my cheek) so GUID is not really an option.
As for #D2, its not slow or hung. In fact, this new drive seems to me MUCH faster the other drives--maybe because it is 7200 RPM instead of 5400 RPM like the others.
Here is a sample of the widget logs. They're all pretty much the same:
Starting standard backup
Backing up to: /Volumes/TimeLapse/Backups.backupdb
Event store UUIDs don't match for volume: Timeless Connections
Event store UUIDs don't match for volume: The Unknown
Event store UUIDs don't match for volume: MasterControl
Event store UUIDs don't match for volume: Music
Event store UUIDs don't match for volume: Entertainer
No pre-backup thinning needed: 493.20 GB requested (including padding), 1.77 TB available
Backup requested by user
Stopping backupd to allow ejection of backup destination disk!
I am not ejecting the disk. It ejects when no one is home. Its working when I leave and ejected when I get home.
Do we need to go to the logs? Here is a sample portion of the system log from yesterday
Jan 20 16:43:18 Master-Control-2 /System/Library/CoreServices/backupd[90769]: Backing up to: /Volumes/TimeLapse/Backups.backupdb
Jan 20 16:43:34 Master-Control-2 /System/Library/CoreServices/backupd[90769]: Event store UUIDs don't match for volume: Timeless Connections
Jan 20 17:57:37: --- last message repeated 3 times ---
Jan 20 17:58:07: --- last message repeated 3 times ---
Jan 20 18:16:33: --- last message repeated 3 times ---
Jan 20 18:17:03: --- last message repeated 3 times ---
Jan 20 18:33:26 Master-Control-2 /System/Library/CoreServices/backupd[90769]: Copied 3032 files (79.1 GB) from volume Timeless Connections.
Jan 20 18:37:38 Master-Control-2 /System/Library/CoreServices/backupd[91797]: Backing up to: /Volumes/TimeLapse/Backups.backupdb
Jan 20 18:37:47 Master-Control-2 /System/Library/CoreServices/backupd[91797]: Event store UUIDs don't match for volume: Timeless Connections
Jan 20 19:39:08 Master-Control-2 /System/Library/CoreServices/backupd[91797]: Copied 30 files (47.4 GB) from volume Timeless Connections.
Jan 20 19:39:24 Master-Control-2 com.klieme.TimeMachineScheduler[92268]: false
Jan 20 20:07:50 Master-Control-2 /System/Library/CoreServices/backupd[92546]: Backing up to: /Volumes/TimeLapse/Backups.backupdb
Jan 20 20:07:57 Master-Control-2 /System/Library/CoreServices/backupd[92546]: Event store UUIDs don't match for volume: Timeless Connections
Jan 20 20:31:42: --- last message repeated 3 times ---
Jan 20 20:32:12: --- last message repeated 3 times ---
Jan 20 22:39:04 Master-Control-2 /System/Library/CoreServices/backupd[92546]: Copied 51 files (118.7 GB) from volume Timeless Connections.
Jan 20 22:39:20 Master-Control-2 com.klieme.TimeMachineScheduler[93729]: false
Do you see anything useful? What's this about the "UUIDs don't match for volume"? I know it is Unique User ID but I'm not sure what it is telling me. What doesn't match?