Why does Time Machine only backup after a reboot? I have a Mini running Lion Server that has a USB hard drive attached to it for backups. I also have a MBP running Lion that I've set to use that same drive as a remote backup destination over the network.

The remote backups have never failed; however, the local backups from Server to its USB disk only work right after a reboot. When the machine tries to run its next scheduled backup it fails with


  Time Machine Error
  
  Unabled to complete backup. An error occurred while creating the backup folder.  
  
  Latest successful backup: 12/2/11 at 11:50 PM


at which point I usually reboot the machine and manually run a backup, which succeeds.

Things I've tried


I've tried googling that error message, but "An error occurred" isn't much to go on.
I've tried repairing the drive in Disk Utility, but it didn't find any errors.


Other Notes

When I first plugged in this USB disk I told Time Machine to erase it and set it up. It has a GUID Partition Table, one 1TB partition formatted to Case-sensitive, Journaled Mac OS Extended.

What can I do to have my backups run without needing intervention?
Things to check:


Software Update (read the description to see if it fixes the issue
for your OS X 10.7 server)
Check if OS X 10.7 Server requires its Time Machine to be formatted
using Case-sensitive, Journaled Mac OS X Extended. Anecdotally,
I've always formatted my Time Machine backup as  Mac OS X
(Journaled) and Time Machines works fine.
Check the Permissions. Check that your OS X 10.7 Server user id has access to the USB partition.