Time Machine Fails

Time machine fails consistently on my wife's Mac Mini. This has been happening from OS 10.7 through OS 10.9.5, the current system. I have been unable to pinpoint the cause, but the problems start when the drive is nearly full.

I did a clean install of Mavericks to eliminate system junk left behind from previous upgrades. I have tried several different hard drives in several different enclosures. I have used both firewire and USB drives. Does not seem to matter, failure still occurs.

Sometimes simply disconnecting and reconnecting the time machine drive will remount the drive and the Time Machine backup will resume. On occasions when it refuses to mount, is invisible to Disk Utility and Disk Warrior, I have to remove the drive and connect it my computer.

My MacPro is running OS 10.6.8 and Disk Warrior has always been able to see the drive and repair it. On returning it to my wife's computer, the drive mounts and Time Machine backups resume. Failure usually reoccurs a few days later.

If I reformat the Time Machine drive, the problems don't start until the drive becomes nearly full, say around 20GB space left on the drive. The current drive is 320 GB and the hard drive being backed up is 86GB.

Does anyone have any suggestions, or is this just a "feature" of TimeMachine that has been overlooked by Apple?