Seems I've run into a potential problem, I don't know whether It's normal or if It's a bug of some form with Snow Leopard. I recently resolved an issue with my old Mac Pro in which the cause was a failing hard drive. Here's the thread. http://forums.macrumors.com/showthread.php?t=1692462 I've notice that a number of recent scheduled Time Machine backups have failed after restoring the OS (10.6.8) to the new drive. I traced the issue to the backup drive being 'full' with backups from Mac OS X from the previous hard drive. - it makes sense that the backup cannot complete a 400GB backup onto a drive with only 100GB free. Quite simply it seems that after the restore Time Machine identifies the old backups as a different machine, thus it can just pick up the backup process where it left off with the old drive. I open my TM disk and the backup is there, it just seems incompatible with the new instance of OS X; I haven't installed any updates since the restore, perhaps a few new files, but nothing large. Has anyone else experienced this? - Whilst the backups are not important since the system restored successfully, it does seem odd that this is occurring. My only real option is to wipe the backup drive, and start a new backup routine. I will also pose this directly to Apple as it confuses me.