If for some mysterious, unknown reason, your backup suddenly stops working and throws VSS errors, it's usually a good idea to check the following:

1. If your server is an SBS 2008 or 2011, and have installed updates, you may have to reconfigure Sharepoint:
   WSB fails on SBS 2011 after updating Sharepoint (0x800423f0 or 0x800423f3).
2. If you get errors about VSS failing to create a snapshot, and you have not changed anything on your server recently, it's a good idea to check the presence of a lot of old shadow copies, taking up space. Although old copies will be purged automatically, the VSS Writer doesn't wait around for it, and fails the backup.
   So try deleting shadow copies or increase the shadow copy storage limit.
3. A backup may also fail because of a full external HD, or when the HD contains incomplete or corrupt backups. Try using another external HD. If it works, the problem is with your external media, and not with your server or with WSB.