This is SBS 2008: We've had this problem for many months now. The backup will work fine for 3, 4 or 5 nights. Then it will fail on the following several nights with the message:
"The server backup cannot start because the drive where you want to store the backup is not available. Ensure that any external drives are attached to the server, and then try again."
When I discover that the backup has failed, I can go into backup and go through the motions of recovering data from one of the good backups. This proves to me that the drive is actually available.
After several nights of failed backups, it starts working again without any intervention on my part. I've tried swapping USB devices around, but the behaviour never changes.
A while back I had another problem which may be a factor in all this.... We replaced the USB drives a year ago. After doing so I found that 3 instances of the backup were starting at the same time. I edited the properties of the scheduled task and set it so that fresh instances couldn't start if the task was already running.
Can anyone help please?