Dissapearing HD

Hey guys, I need some help pertaining to dissapearing drives.  My friend's 10GB HD decided to dissapear from his computer, he's running WinXP service pack 2... and that's really all I know of his computer.

Anyway, besides the fact that the HD might have died on him what else could be the problem? Is there some kind of tool that'll search for the drive or something? :\

Never dealt with this before... Any help would be greatly appreciated!

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Interesting, he did just add a wi-fi card to his computer so that might be it... I'll check and see, also I can't really tell if the drive is spinning because it's right next to his other drive... that works... You guys think I should try booting up just that drive? Crap... he doesn't have Windows installed on that drive though... Well, I'll see what I can do. Thanks for the help!