Consolidate Backup Archive behaving strangely

Here's my setup:
- Backing up to an external drive
- Daily Image Backup
- Automatically consolidate when size or archive exceeds 315GB

Things went smoothly from the time I started using it, in April, until a couple days ago. Then I noticed the drive constantly grinding. Sure enough, there was a consolidation in progress.

All of my numbered .tib files, dated starting in April, seem to have been rewritten, since they all now sport a date of 8/31. They range in size from a few hundred MB to about 35GB. There is also a non-numbered .tib file (MyBackup.tib), also dated 8/31, which is 117GB.

According to the status bar, the consolidation was at 66% on 8/31, with many hours to go. I let it run. On 9/1, yesterday, it said it was at 67% (and the nightly backup had not run). A new file had been created, which looked like a temp file, showing at 0kb and dated 9/1.

Today, it was back to 66%, with many hours to go - and no daily backup last night. The temp-seeming file was still at 0kb. I killed the consolidation and ran the daily.

Any ideas about what's up?