Time Machine Backup Interrupted. Im using a Sony External Hard drive

i have a white macbook, version 10.6.8. I wanted to back up my mac so i connected my Sony External Hard Drive and used time machine. About a quarter way through the back up, i get this message saying to "You need to restart your computor. Hold down the power button until it turns off, then press the power button again." I cannot press/move the mouse in anyway or do anything when this message comes up. When i open my mac again a window comes up saying "You shut down your laptop because of probelm". The backup stops and fails. This has happened 3 times now and has never happened until i tried to back up 😕. What seems to be the problem and what should i do to fix it?