Time Machine makes MacBook Pro freeze completely

I have used Time Machine to backup my data on an external hard drive with ample space for the past year. However, for the past month or so when plug in my hard drive and try to back up my files using Time Machine, it completely freezes my computer, to the point where I need to hold down the power button for 10+ seconds and completel restart. At first I thought this might be due to not enough RAM, but discovered it does this even if all other programs are closed.

Any advice is greatly appreciated!