why does my computer slow down when I connect an external hard drive

I've been using a 2TB western digital external hard drive with USB connection. It was working fine, but once it was up to 1.95TB, my macbook pro froze up. Looking at my activity monitor, my memory dropped to nil once the hard drive was connected. The same hard drive works fine on my PC and I have no issues accessing all the files, just not on my mac. Any suggestions?