I cannot back up to an external hard drive since I installed OS X Yosemite

Since I installed OS X Yosemite I cannot back up to an external hard drive. I get the message, "Preparing backup" that can last a long time, then if I leave it for hours, it eventually backs up, but only 20 mb after 5 hours. Help resolving this will be greatly appreciated. It's been 46 days since my last back up to the external hard drive.