I had a few externals doing stuff like that. There are 2 repeating fixes that I try first.
1. Get all the files off the drive then delete the partitions, create new partitions and quick format the volume.
2. Replace the USB cable.
I'm having some trouble with my WD Passport external HDD. The model is a WD500BEV. I use the drive to transfer files from my work computer to my home computer, and lately I've been having a number of CRC errors. I've checked the drive with a few programs and it's passed all the tests. Could it be the cable? Is the data on the drive bad? What could be causing all these erorrs?