If the disk does not show up as a drive in disk management it may be a USB issue.
How are you plugging the drive into the usb port?
If you are using just a cable from the HDD to a USB port, the problem is most likely insuffient power to HDD.
USB 2 ONLY provides about 2.5 Watts (0.5 Amps @ 5V), and most HDDs require more than that ar startup and when reading and writting to the drive.
.. For external HDDs I always use a Powerd USB hub between HDD and USB 2 port. This provides up to 2 Amps or 10 Watts.
.. Alternative is to use a two headed USB cablethat will plug into 2 usb 2 ports. Note Prefer the previous method.
Added: USB3 provides more power than USB2. So most USB3 external 2.5" HDDs wil work when plugged into a USB 3 port.
I have the same problem but in my case, my Toshiba external hard disk is not even showing in the disk management. Actually it was working properly a week ago but now when I connect it on my HP laptop its make the sound that is connected and even the light on the hard disk is on but its not showing anything. I don't know what to do