I am running SBS2008. We are having ongoing problems with our backup. We back up to 2 identical external hard drives and the schedule is configured to run once per day every day. On a frequency of between 3 and 14 days the external hard drive which we change daily except for weekends will go offline. When I look at the device manager it will say that there is an unkown device in the USB field. I have tried deleting the unknown device and restarting the external hard drive as well as restarting the server. I can usually fix it but the fix is not consistant. Last week I deleted the unknown device and 1 USB port and that fixed it, today I had to power reset the External HDD. I moved the External HDD's to a different computer and everything appears good. I can see the backup and navigate around the drive.
I tried deleting and re-establishing the schedule, hoqwever, the problem with that is SBS wants to reformat the drive so previous backups are lost. The External HDD's are 320 Gb SATA and we are backing up about 110 Gb so there is no space issue.
What could be causing backup problems like this and what would be a good way to fix them permanently?