Windows backup stalls when creating a system image.

My laptop has been backing up once a week fine (except for an error about a missing file that is on the target drive, not the backup drive) for months. In the past few days, it has suddenly failed with repeated attempts to backup, it always stalls at 57% while creating a system image. I've tried a clean boot, same results. I'm backing up to a network drive (USB hard drive attached to the router), 274GB free of 458 total, and the total backup size has been pretty small as no heavy data is kept on the computer.

Lenovo SL510
Windows 7 Professional

Any thoughts on why the sudden failure?

I had this problem, and following advice on another forum, I plugged my Usb3 portable hard drive into a Usb2 slot instead. It then worked, don't know why!