I just bought a Seagate Barracuda 400 GB internal HDD and created a external USB drive by enclosing it in a USB casing. However once I plugged the HDD in my laptop, I did not see my Drive listed in the My Computer window, though the regular messages like "A High-Speed USB drive has been connected to your computer", and " "Your hardware is ready for use", were shown, but it did not appear in the Device Manager list. I was worried as this mechanism of converting an internal HDD into an external USB one was supposed to be plug-n-play. A little bit of googling game me the solution which I provide the below:
Solution: Right-click on My Computer. Select Manage in the pop-up menu. In the Computer Management window, select Storage, then select Disk Management. In the Disk Management window, you should see a list of available storage devices. Look for the Disk that has a capacity closest to your External Hard Drive. Right-click on the right-hand box; in the pop-up menu select Delete Partition. Once you do this, it will say “Online" & "Unallocated”. Right-click on the box and select Create New Partition in the pop-up menu. When the Partition Wizard appears, select Primary Partition and click on Next. You will see a default value for the Drive; click Next. You will then see a drive letter (you can change this drive letter if you wish); click Next. You will then be asked to format the Drive. Once done, you are good to go.