Only one backup set after nine months?

Hi,
I've used Windows Backup and Restore ever since I installed Win7 on this machine. There is a 750gb drive backed up to a 1.5tb WD Elements drive. I have backups set to run early Monday mornings.

Yesterday, Backup told me that my drive was full and there was no room for another backup. "manage space" told me that there were two backup sets, and one system image. The system image was 615gb. The datafile backups were 700gb, and there were two. The big one was from 5/08/11-2/20/12 and was 530gb while the second was from 2/20/12-2/20/12 and was 170gb. There were 80gb of free space.

I figured the second one was not a complete datafile backup and decided to throw caution to the wind and delete the earlier set, and then run Backup again, which it completed Tuesday afternoon. This resulted in a single datafile backup set from 2/20/12-2/21/12 which was 419gb. The system image is now 619gb, and there is 359gb of free space.


I always thought I was supposed to see multiple backup sets, but now I'm thinking it has no room to make a completely new backup set, so it just keeps appending deltas to the original set until it gets full. In that case I probably wouldn't see additional sets unless I was using, say, a 3tb drive. Is that correct?