Sometimes I leave my WD(Same as yours) and decided to go to My Computer, it doesn't load up but when I hear a clicking sound from it, it loads up. Maybe if you wait for a few moments(Maybe a minute) then it will solve the problem. Also, I found out using a extension cord for my USB can cause problems.
When ever i connect my WD 160GB Passport Portable (black) I am unable to access it through My Computers, while the device manager does. I am running Windows Vista on a HP Pavilion DV6000. The same thing happens when i connect it to a HP Desktop running Windows Vista (media). Also, I was able to access it before. Any advice?