Windows 7: External Drive no longer accessible

Hi,



I have a 500GB Toshiba Canvio External HDD which has worked marvelously for about a year now. I keep all my music and school work on it and have never had problems accessing anything. In the last week, however, I've been unable to access it; it doesn't show up in my computer, device manager, or disk management. However, it does work in my friend's computer and when I plug it into my own computer, it hums and runs like normal. The light turns on and there is no steady skipping and I haven't dropped it so it's definitely not broken. Any thoughts? Thanks