Hi everyone! I had a similar problem to what a bunch of people here said. I have Windows 7 and was using a Toshiba external hard drive.
So, the external hard drive made a sound when I connected it. BUT
* When I plugged it in to any of my USB ports, I got this standard message that the device wasn't recognized and it may be defective.
* It didn't show up on My Computer
* It didn't show up in Disk Management
I was pretty worried. Searched all over. Finally put together a couple solutions and solved it! Here's what I did
* I went to the Device Manager (you can just search it on your computer and you should find it)
* From there, I went to/expanded Universal Serial Bus controllers.
* There's a whole list there of all different devices and stuff, but one of them there was titled "unrecognized device", or something like that. I opened that and went to the Drivers tab, and there, I uninstalled the driver. It gave me a warning before that I was deleting the device from my computer, so if you get that, don't worry, just do it.
(*After I uninstalled the driver, I then clicked the button to update drivers... but it said the computer already had the latest driver installed. That kind of confused me because I thought I just uninstalled it, but either way, maybe for you it doesn't update drivers automatically and you'll have to do it there. You need the latest driver for it to work. I guess for me that step was unnecessary.)
* My external hard drive now showed up on Disk Management!
* Now, from Disk Management, in the top section where it just says the list of your devices, I right clicked on the hard drive, and clicked on Change Drive Letter and Paths...
* From there, I clicked "Change". My device was under drive E:, and I changed it to F:, because I saw some people saying online that you have to change the drive there to another one and then it'll work. It did for me! My hard drive now showed up in My Computer.
Hope this works for you!