Infor on WD External Hard Drive temporarily disappears

I recently purchased a WD External Hard Drive 320 GB for storage to clear my very full hard drive. I plugged it in, as directed and copied my files over. All went smooth, till I could not view them anymore (but they are there). And, it must be unplugged upon start or my computer freezes. I was advised to move my entire hard drive over (DriverImage XML). All went well. Not a usable view for me to use, by my documents (photos/music) is all I really care about. I start my computer, then plug in the WD....all is there, but after a short period of time, I am once again unable to use/view the files. The drive is recognized. I am very confused! The directions, nor website help. I have yet to delete from my hard drive because of this. Also, I believe it is always right before they disappear, I get a message saying I/O Error. Help!!!!!