The partition table may have been deleted.
Go here: http://www.cgsecurity.org/wiki/TestDisk
and download a free utility called TestDisk (I downloaded the latest NON-beta version). It's a zipped file, just extract all files to a folder of your choice leaving the option "Use folder names" checked. Then, go to the folder where you unzipped the files, go to the Win folder, and run the testdisk-win.exe file. It runs in a DOS window, even though it says it's a Windows app, but it doesn't really matter at all.
So, start the app (you must run it with administrator privileges), wait for it to recognise all the drives, it will take 5-15 seconds. Choose the external drive. Confirm Intel/PC partition. Analyse it. Proceed. Enter to continue. Write on the next menu. Confirm. Quit the app. Disconnect USB cable from the device following the safe hardware removal procedure. Reboot your pc. Re-connect the device and it should be properly recognised, and the whole thing doesn't take longer than a couple of seconds!