We are using Windows Backup on SBS 2008 SP2 and backing up to 1TB external hard drives. Recently after switching drives our backup started failing because the backup drive is full and auto-delete isn't automatically deleting older backups/show copies. I'm trying to get more information to help me effectively prevent this problem from reoccurring in the future.

How I can tell that the drive is getting full:
In the event viewer under Windows Logs > Application, I'm seeing Event ID 517 but it fails to show an intelligible description. However, under Applications and Services Logs > Microsoft > Windows > Backup > Operational, I'm seeing an event with the ID of 5 and a description like this: Backup started at '10/4/2011 12:30:12 PM' failed with following error code '2147942512'.

One of the most informative posts I've found on this error is located on Microsoft's Technet Forums here. In that post, a Microsoft representative gives this hazy explanation:

auto-delete feature to ensure that at least some old backup copies are maintained on the disk -- does not automatically delete backups if space utilization by older copies is less than 1/8 of the disk size or in other words, 13% of the disk size. that means if the one full backup copy does not fit in the 7/8 of the disk size, backup may fail with disk full error. auto-delete will not automatically delete older versions to reclaim more older versions of backup.

In the above explanation, I do not understand what is meant by "older copies" except that it appears that anything older than the very last shadow copy would be considered "older copies". I'm going to make the assumption that this problem where auto-delete will not work will affect any hard drive that is large enough to make an effective backup drive, or in other words, any hard drive that is large enough to hold more than one backup/shadow copy at once.

The same MS representative proposes the solution of using a larger backup drive. I can't understand how this will help. It appears to me it will simply delay the problem until a later date.

In order to resolve this problem for now, I did the following:

1. Assign the backup drive a disk letter under disk management.
2. Run the command line with Administrative rights.
3. diskshadow.exe [enter]
4. delete shadows oldest x: [enter] (where X: is the letter you assigned your backup drive)
5. I manually ran the above command some 60 or 80 times to free up about 200 GB of space on my 1 Terrabyte External Hard drive.

However, I do not feel this is a satisfactory solution to prevent the problem from happening again in the future. Does anyone have a solution to prevent your Windows Server backup drive from getting full?