|If your external drive has a 3.5" hard drive inside of it (is NOT a "portable" drive) and came with a AC to DC power adapter, you must use that power adapter with the external enclosure and it must be working properly.|
There are three common scenarios for your problem.
You don't have the external hard drive's USB cable plugged into a USB port that can actually supply the full max USB spec 500ma, or the USB cable you are using did not come with the external drive and is inadequate.
E.g. There are lots of laptops that can't actually supply 500ma per port when more than one USB device is plugged directly into the USB ports built into the laptop, but an external hard drive will work fine when plugged in BY ITSELF when it's plugged directly into one of the USB ports built into the laptop.
In that case,
- the operating system may not even see the external enclosure's circuits or the hard drive inside of it at all, and you may not hear the usual dong-ding when you plug it in while the computer is running.
- the external hard drive will work fine and the partition(s) on it will be recognized as drive letters fine when you plug it into SOME USB ports on SOME computers, if the problem is not caused by an inadequate USB cable.
Troubleshooting USB device problems including for flash drives, external drives, memory cards.
See Response 1:
If the partition(s) on the external hard drive do NOT show up as drive letters when you plug it into ANY USB port on on ANY computer, assuming that the USB cable you are using is adequate, then you probably have one of the following two scenarios.
You DO have it plugged into a USB port that can actually supply 500ma, there is no problem with the USB cable, you DO hear the usual dong-ding when you plug it in while the computer is running, but the partition(s) on the hard drive DO NOT show up as drive letters in Computer (Vista and Windows 7) or in My Computer (XP and below) or in Windows Explorer.
In that case data on the hard drive has been damaged and the drive will NOT read properly when connected to ANY computer.
The drive enclosure's circuits will show up as being recognized in Device Manager - USB controllers as a USB mass storage device, and the hard drive inside the external enclosure usually shows up fine in Device Manager - Disk Drives and in Disk Management.
In that case, you need to use a program that can repair the data damage without you losing any of the data on the drive, or without you losing most of the data on the drive.
If you have not dropped the drive or otherwise damaged it, your problem was probably caused by you NOT clicking on the Safely Remove Hardware icon in your Taskbar and STOPping accessing the drive BEFORE you unplugged the drive while Windows was running.
There are several programs that you could try that will probably fix your problem.
Specifically, links to some programs that can repair the data damage are here -
see response 2:
You have dropped the external hard drive or have otherwise physically damaged it.
In that case, the external enclosure's circuits, or the hard drive inside the enclosure, or both, are damaged.
In some cases removing the hard drive and installing it in a new external enclosure will fix the problem.
see response 2:
That post also has info about testing the external enclosure's hard drive. Apparently, the Windows version of Seagate's SeaTools can test any hard drive brand when it's in an external enclosure.
Do the long test.
You can also remove the hard drive that's inside the external enclosure and connect it one way or another to another computer and test it with hard drive diagnostics. E.g.if the laptop (2.5") sized drive is SATA the sockets on the drive for SATA data and SATA power are identical to the same on a desktop 3.5" SATA hard drive, so you can easily connect it to a desktop computer.
Don't boot from your laptop hard drive on the other computer - the operating system will probably not load all the way if the mboard on the other computer is more than a little different than the one on the laptop - that's normal.