That is confusing. If the external drive was not formatted, it could not have a drive letter. If it doesn't have a drive letter, you can not move or copy anything to it. You can not even see it in Windows Explorer, literally. That is confusing, please clarify. Also, it is extremely unusual to see an external drive that is not formatted when it comes from the factory. You should not need to format it.
It is impossible to go back now and recreate the chain of events, but I think something else happened. You didn't say, but have you put the external drive back on the original computer? If not, try that.
You used the term "unplugged from windows explorer." Sorry to be picky, but it is important to get the terms right since we are only reading what is written, we have to intrepret it using terms that we know. Do you mean that you plugged the external drive into a machine that is running Windows 7? You can not "unplug" a drive from "Windows Explorer," there is no plugging and unplugging because that is software. If you mean you unplugged it from the computer's USB port, then I would ask: did you do an "eject" first? It is important to eject a USB drive before unplugging it.
Again, the terms are critical because we can't see the problem and because it is so hard to discuss back-and-forth in a forum.
If you really need to re-format the drive, plug the drive into the USB port.
click on:
Start
Control Panel
Administrative Tools
Computer Management
Storage
Disk Management (Local)
and you will see all the drives that are in the machine plus the external drive. You should see that the external drive has some kind of partition, or no partition at all. If there is a partition, delete it then create it again. If none, create a new one. Now format with NTFS. The drive should now be useable.