1. On the DC, open the command prompt and run "vssadmin list writers". If there is an isuse with any of the writers, reboot the server.
2. Run another backup and if the issue is not yet resolved move onto the next step
3. Open the Server Manager on the DC and then expand Storage and click on Disk Management
4. Verify that the System Reserved partition (typically 100mb) is in a healthy state with a proper volume label and NTFS file system and that it is online. If there is an issues with the system reserved volume then this could cause the issue.
5. The below steps may resolve the issue, although this is up to the customer at this point to perform the resolution in their environment:
6. From the command prompt run DISKPART with the following commands:
C:\DISKPART> List volume
C:\DISKPART> Select volume <number of 100 MB system partition>
C:\DISKPART> Online volume (if volume is showing offline)
To prevent volume from going offline in the future, refer to Microsoft KB#2419286
C:\DISKPART> automount enable
7. Reboot server and run DISKPART again to verify that the volume is not offline again
8. Run backup again