I have a 1TB external drive, and about 400GB total space used on the two lettered drives in the machine. The backup has 157 backups on the external drive before the backup started to fail. The Windows Server Backup program makes an initial backup of everything on the covered drives. It is extremely compressed. After that initial backup, it is just a series of incremental backups that are done. Hence the ability to have so many backups of the amount of data we are talking about here. According to the quote from an article on the MS site about how it is supposed to work, it is not working.

The crux of the issue is that the backup cannot create the new file needed to hold the data for the next backup. If it is supposed to automatically manage the backups and create the space needed, why is not the space being created?

If this were an issue at a client site, I’d have a new drive to put in place for the backup and MS on the phone to determine what the problem was with the backup looking for a resolution. Since it is my own server, and I do have other backups of critical data, I am not so concerned. I do have certain ideas as to why the problem exists, I am looking for some validation for any of those ideas, and, as I have time, will experiment further to prove or disprove those ideas.