My 2 1/2 year old Mac Pro required a logic board replacement to fix a failure to start problem (Applecare just paid for itself). When I got the Mac Pro home and booted it up, a Time Machine message immediately appeared asking to back up to my external hard drive. Seemed reasonable so I let it happen. A progress bar appeared showing that Time Machine was backing up over 400 GB, and it took 3 hours to complete. When completed, I opened Time Machine to fine that it had over-written all the previous back ups. Not having over a year's worth of back ups was bad enough, but it was made worse by the logic board's replacement causing problems that could have been easily fixed if I had a previous set up to restore. When using my Mac Pro after the fix I discovered that the drivers for my 2 printers, flat bed scanner and Parallels were gone as well as iPhoto themes which prevented printing from iPhoto. After much time and cussing, I reinstalled all the missing drivers. My Apple repair technician who was sympathetic to my problems but unwilling to agree to send someone to my home to fix the problems did agree to provide a CD with iPhoto themes for me to reinstall. The lessen learned here is to not have Time Machine do a back up until a repaired computer is fully checked for problems.