My normal experience is that the built in backup for SBS 2008 generally just works … that is, until now. I returned from vacation to find backup errors on an SBS 2008 customer’s server. The error message was: “Failed – There is not enough space on the disk”

So, I hunted around for a solution to get things running, and here’s what I found that worked for me. What we will be doing is to manually delete the oldest shadow partition(s) from the USB drive, thus making room for newer backups. Here’s the process we will follow:

1. Temporarily assign a drive letter to your USB drive being used for backup
2. Use the DiskShadow utility to delete the oldest shadow partition(s) to free up enough disk space
3. Remove the temporary drive letter assignment
4. Rerun the backup

So, let’s get started:

1. Open up Disk Management (Start –> Run –> DiskMgmt.msc)
2. Right click on the USB Drive being used for backups, then click Change Drive Letter and Paths
   
   Remember: by default, the USB drive being used for SBS Backups is generally hidden.
   We can, however, assign a drive letter temporarily to make room on the drive.
   
3. I mounted my USB drive to X:
   Keep the disk Management window open, as you will be needing it.
4. Next, open up a command prompt with administrator rights
   1. Type: diskshadow and press enter
   2. Type: delete shadows OLDEST x: and press enter
   3. Repeat the delete shadows command several time until you have freed up enough disk spac
   4. Then close the command prompt window
      
5. Return to the Disk Management window, click Change Drive Letter and Paths… and remove the drive letter assignment. 
   Note: removing the drive letter may take a few minutes, so do not panic!
6. Next, rerun your scheduled backup, and hopefully your backup will successfully complete!
   

Finally, you will find some helpful information in this blog post from the Microsoft Storage Team on backups and space management for Windows Server 2008/2008R2.