Windows Server 2012 Essentials backups are designed to manager their own space on the backup drive but that doesn't stop the system sending an alert when space left is beow 10%

This  note details how to manually delete snapshots from the server backup disk (the USB drive)

1. Open an elevated (as administrator) command window
2. Enter the command "Mountvol<cr>"
3. Look for the drives that have a description of *** NO MOUNT POINTS ***, make a note of their Volume names
4. Now enter "vssadmin list shadoes /for=[volumename] e.g. \\?\Volume{a6ebc3b1-3fba4ca-8e90d23dd23a}
5. Identify, from the date and times of the snapshots taken which one of these volumes actually represents the scheduled backup to the USB drive
6. Enter the command "Diskahadow.exe <cr>"
7. At the prompt enter the following command to delete the latest snapshot
8. delete shadows oldest [volumename] e.g. \\?\Volume{ecce3d21-5c0b-416c-b5a1-37928a2cfb6e}
9. Repeat approx 20-30 times, for the number of snapshots to be deleted
10. Finaly enter "exit"
11. Changes will not take effect until after the next scheduled backup