Same thing has happened with my HD-PCT500U2/B-US.
Before it started this, it would upload the files very slowly. The videos I had on the HD were dragging along and it took a while to upload a simple MS Word document. Suddenly it just stops showing up on my computer.
When I go to device manager on my PC the buffalo drive is there and it says "its working properly", but it doesn't show up anywhere else and it's not working on my PC. The HD clicks, the fan comes on, the blue light blinks for a second and then stays on, but it doesn't show up. HELP!