Backing up client machines to a specially-provisioned network share is one of the marquee features of Leopard Server. Unfortunately, it’s a feature with far more promise than documentation. When it works, it’s a dream, finally freeing you from an aging Retrospect setup (or worse). When it fails, though, it tends to do so without much insight as to what’s gone wrong. If you can’t get Time Machine backing up to a server, you’ll get plenty of detail as to why. Once a machine stops backing up properly, though, there’s no real indication as to how you might get it working again.

There are a variety of reasons your network Time Machine setup might start excluding certain machines. Rather than address them each individually, the following is a general troubleshooting approach for the full gamut of issues. Follow these guidelines, and your Time Machine setup should be working again in no time.

First, turn off Time Machine on the misbehaving client machine. With the large, decorative switch now in the “off” position, remove the preferences file at Library/Preferences/com.apple.TimeMachine.plist. If you’re dealing with a corrupt preference file, that should resolve the issue immediately.

Next, remove the Time Machine “cookie” from your network share. These invisible files are named with a period, followed by the MAC address (also called the ethernet id) of the “Ethernet 1″ interface on each client machine (not necessarily the network interface you’re actually using). If the machine has a primary MAC address of a1:b2:c3:d4:e5:f6, then you’re looking for a file called .a1b2c3d4e5f6 at the root of the share. Get rid of it, and any identification issues with your Time Machine client should disappear.

Finally, you’ll need to reintroduce your client machine to the Time Machine server. In order to access your share every hour, Time Machine stores authentication data in a keychain when it’s first configured. Delete all references to your Time Machine server from the “System” keychain (most simply with Keychain Access in the Utility folder). Once no memory of the name or IP of the server remains, you can reconfigure Time Machine to back up to your server again.

These steps won’t solve every Time Machine server problem, but this quick checklist can often get your users backing up again, and get you back to working on larger problems.

Recommended Reading: For more information about common backup problems, take a look at Apple’s own Knowledge Base on troubleshooting Time Machine issues.