If you are using Server 2008’s built-in “Windows Backup” tool to backup your server, you may run into a problem where the “auto-delete” function does not delete your older VSS copies if your backup destination is not large enough for the disk(s) that you are backing up. Then you will run into failed backups because there is not enough disk space to create the backup copies.
This post from a MSFT explains further (taken from: http://social.technet.microsoft.com/Forums/en/windowsbackup/thread/c55e4e16-22af-4d84-b2c4-7aedcf512469)
auto-delete feature deletes the older version of backups on the disk by deleting the corresponding shadow copy. each backup version is maintained in a a volume shadow copy on the backup location. shadow copy of a volume is a readonly copy of the volume. there can be multiple shadow copies on a volume. these are maintained by tracking the changes on the volume and storing them in what is called diff area files on the volume.
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.
backup will fail with 0x80780030 media is full error.
it is recommended that
1. either backup disk with bigger size can be used or
2. vssadmin utility can be used to delete more older versions of backup if they are not required.
vssadmin list shadows /for=x: will list the shadow copies on x: and creation time.
vssadmin delete shadows /for=x: /oldest can be used to delete the oldest shadow copy on x: where x: is the backup location.
diskmgmt.msc can be used to assign drive letter to the backup disk temporarily and again removed.
vssadmin.exe tool need to be run from elevated command prompt.
You’ll need to use “Wbadmin get versions” to see what’s been backed up, and to what device.
Then, you can delete the oldest copies of your backups using the “Delete shadows ID <Shadow Copy ID>” command