De-powering the external drive, the computer, or both may work.
Re-installing your operating system may work.
There is another possibility...
Are you using a desktop or laptop? disregard the rest of my post if you're using a laptop.
[sblock]Does this external hard drive:
weigh at least one pound?
have a power connector in addition to the USB connector?
If the answer is "no" to either question, disregard the rest of my post.
[sblock]there should be some screws and a small sticker on one of the sides, possibly the back that says "warranty void if removed". This sticker usually covers a screw. You will need a phillips [or "x" tip] screwdriver, and a powerful flashlight.
NOTE: make sure to discharge your body's static electricity before even thinking about doing this.
FURTHER NOTE: this will void your warranty on your external hard drive; you've been warned.
FINAL NOTE: make sure, just in case, you've got all your important stuff on your computer backed up somewhere, like on dvd's, flash drives, or something.
[sblock]0. try going to tom's hardware, or some other websites specifically for PC troubleshooting and repair suggestions before you take my advice. More opinions are good things.
1. unscrew all screws, and separate top/bottom or left/right parts of the case.
2. you're going to see the hard drive hooked up to a little circuit board that's usually at the 'butt' end of the hard drive; the board is where the USB and power connector plug in.
3. without disconnecting any wires, look at the back of the hard drive itself; verify if it's an IDE connector or a SATA connector:
IDE connector - Google Image Search
SATA connector - Google Image Search
4. discharge your body's static electricity again, just in case; open your computer's case, and verify you have the same type of cable, and a free end [nothing's hooked up to it].
If it's IDE, you may have more than one cable; if you do, try to find the cable that's already plugged into your main hard drive, follow it along, and find the other connector on the same 'ribbon'.
IDE ribbon - Google Image Search. You may have to unplug a cd/dvd drive in order to use this particular plug. DO NOT unplug your main drive.
[See also point 5] If you've got mulitple drives in your case, you'll probably have to get a dentist's mirror or one of those bendy mirrors mechanics use, as well as a flashlight to look at the jumpers of each drive to see which one is set to 'master' or 'cable select', thus determining which drive is your main, or usually "C drive". USUALLY the main drive's jumper is set to 'master' or 'only drive'.
Master, Slave and Cable Select Hard Drive Jumper Settings - PC Hardware - Find Free Computer Help Tips Online
If you have SATA, you'll probably need to either run without a device, such as a cd/dvd drive, or you'll have to go down to circuit city or best buy to get one:
SATA cable - Google Image Search Before you buy a cable, make sure you've got an extra plug on your motherboard to plug it in.
5. back to the external hard drive; there's probably going to be five pairs of tiny pins in the back with a little 'jumper' on some of them; the brand of the drive and the instructions for jumper positions are also probably going to be on the drive itself. Go to the manufacturer's site to get more information on the drive and jumper positions if at all possible.
Hard drive jumper - Google Image Search
6. power down computer; keep it plugged in. discharge your body's static electricity; set the jumper position to 'slave' on the external hard drive.
7. you need to apply power to the hard drive through the computer's power supply. your power connectors will probably look like this:
http://pictures.xbox-scene.com/xbox360/360sata/Molex-style-power-connector.jpg
If not, look for some of these:
hard drive power connector - Google Image Search
8. checklist
discharge your body's static electricity one last time
verify the jumper on the external drive is set to 'slave'
verify power connector is plugged in
verify SATA or IDE connector is plugged in
If IDE, verify the external drive is plugged into the same 'ribbon' as the main drive, and the main drive's jumper is set to 'master'.
9. power up computer and access drive. NOTE: some anti virus software will want to scan this drive, because it's 'new'; i.e., "drive D wasn't there yesterday" or "it was a cd burner a little while ago" etc...you get the picture.
NOTE: always discharge your body's static electricity before touching stuff in the case!
[sblock]10. don't EVER tell ANYONE you did this, not your family, your SO, and ESPECIALLY your friends, because they'll think you're some l33t h4xx0r d00d and will want you to fix their stuff for free
11. [optional] call best buy's 'geek squad' [or any computer repair place where you live] and tell them you've got an external usb hard drive that's not accessible anymore from your computer; ask for an estimate on how much they charge to diagnose AND repair this problem.
Take the quoted amount and spend it on yourself or your loved ones after patting yourself on the back; you deserve it
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