Honestly It's the External mixed with your 3 year old machine.
From time to time the USB controller won't recognize newer technology based External Drives. Unfortunately this is a knows issue with Microsoft, and this happens alot (especially with Ipods for some reason)
You can try updating your USB controller driver, but it's not 100% certain that will work.
Does it work specific times, and NOT work other times?
Have you noticed any steps where it works better than normal?
ex: right after boot up, or anything like that?