I’m bored of sending comments about this to posts in LiveJournal’s macosx community, so I’ll make a full post about it.
Did you try the defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1 hack to backup to a network drive?
If so, you’re going to hit a brick wall and your backup will be useless very soon (if not already)!
Simple explanation: Once your backup disk fills, Time Machine accidentally corrupts the backup.
Technical explanation: Volumes that are otherwise hidden unless you run the above command in a Terminal aren’t guaranteed to be formatted with HFS+. To get around this, Time Machine creates a sparse bundle disk image on the target drive, and formats it HFS+. It’ll then mount that disk image whenever it runs a backup. The problem occurs when the disk fills up. When OS X deletes files from a sparse bundle disk image, it tries to free the space that was previously allocated to these files. There is a bug in the code that does this that destroys the disk image completely, making your entire Time Machine backup useless. Use SuperDuper until they fix it, it can backup to sparse images (as opposed to sparse bundle images).
So that’s why you can’t back up to SMB / AFP / NFS shares with Time Machine for now. You can let Time Machine try, but it DOES NOT WORK (yet).
Fingers crossed this gets fixed soon.