I'm not sure this might be the cause, but you should make sure that all connections, specially the IDE cable (that fat one like a film attached to the center of the driver).
Check both ends of that cable: the end of the driver and the end at the motherboard. An unstable connection may cause this kind of failures and could harm your system as it's not prepared for plugging and unplugging IDE drives while being on.
You should also check out the power cable (the mid-sized plug with 4 holes) just in the case the drive doesn't respond to eject or stops working suddently (watch its LED).
You might also have a virus. Get to
http://www.activescan.com to scan and heal your computer for free online, if you don't have any AV installed yourself.
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