Time Machine failure

Time Machine has ground to a halt. It starts working, then after backing up a very small proportion of the changes it freezes for hours. I end up cancelling the backup because it's going nowhere in my lifetime. It's exactly the same backup (USB) disk that has worked perfectly for 12 months or so.

(The very first backup of the whole hard drive only took about 5 hrs.) Now, you can leave it on overnight and it's at exactly the same point that you left it!

I tried dumping the time Machine Preferences - no change.

Does anybody have an idea? Mac Help does not seem to have any.