There are several possible problems here. None of them are particularly simple to diagnose or resolve, but I’ll run through some of the ideas that I have.
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First, back up
I certainly have to start with my usual admonition. If you’re about to lose some data if you can’t recover this drive, then you weren’t backed up. I hate to say that you’re asking for trouble, but in a way, you were. Drives fail. It happens and you have to be prepared.
With that out of the way, let’s see what we can figure out.
Troubleshooting the drive
With the drive plugged in, fire up the disk management console. Right-click on My Computer, click Manage, and then click the disk/drive manager. See if the drive is listed at all and what its status is.
The drive may not have a drive letter associated with it. The problem could be as simple as that having gone away for some reason. Look for other ways to identify the drive, like the label or size of the drive.
If it says unformatted or uninitialized, then there’s probably a content problem on the drive itself. Fixing it will be difficult. I have a solution for this that I’ll talk about in a minute, but it’s not going to be pretty.
If it looks okay, you can then right-click on that drive and see if perhaps you can just assign the drive letter to it. That could be enough to make the drive reappear.
What if the drive is still inaccessible?
Because you’ve taken this to other machines where it also doesn’t work it’s likely that things are a little bit more serious. If it doesn’t show up, particularly on multiple computers, then I actually suspect that either the external drive electronics or perhaps the circuitry on the drive itself is somehow faulty.
The next step would involve extracting the drive from its enclosure and connecting it directly to a PC. Now if that’s too much for you to do, then it’s probably time for you to see if you can find a technician.
While that drive is directly connected to a PC without the USB interface but rather through a SATA or PATA connection (depending on the type of drive it is), you can then repeat those drive manager tests that we did above.
If it works, then you know it was the USB enclosure at fault and you can then just replace that or just install that drive as a second drive in your PC permanently as it’s there already.
If the drive is still not accessible, then there’s probably a problem with the drive itself. If you’re trying to recover data from this drive, then it’s time to consult either a technician or a data recovery service.
But, if you actually are backed up and this doesn’t represent any data loss for you, then it just becomes a simple matter of replacing your external drive with a new one.