Hi James and welcome to the community.
Sorry to hear about the issues you're having with your external HDD.
Basically taking a drive out of its original enclosure is never recommended, but since there's no turning back now, the least we could do, is to try and figure things out starting from where we are right now.
Did you experience any other issues with the HDD? Any unusual sounds coming from it, disconnecting randomly, "freezing" your system, etc? Do you need to recover data from it, or you want to try and make it work as an internal HDD?
If you don't need any data out of that drive, then reformatting the HDD might restore its original capacity. Also I'd recommend that you download DLG (Data Lifeguard Diagnostics) and run both tests (Quick and Extended), to see if anything unusual pops-up. Here's how to do that:
How to test a drive for problems using Data Lifeguard Diagnostics for Windows.
On the other hand, if you need to recover data, you could try that by using data recovery software:
http://www.tomshardware.co.uk/forum/id-1644496/lost-data-recovery.html and http://pcsupport.about.com/od/filerecovery/tp/free-file-recovery-programs.htm. Another option would be to try and access the drive via Ubuntu Live USB/DVD, to see if it's properly recognized and if you can get to your files. However, for the most reliable data recovery option, you'd have to go for a professional solution, such as a data recovery company:
WD Data Recovery Partners.
Hope that helps. Please let me know how everything goes.
Boogieman_WD