Weird bootcamp partition/external drive problem on MBP?

Hi,

I was trying to create a Win7 partition on my 500GB G-Drive external but weird stuff has happened to it since I last used it. First of all, it displays it as a 750GB drive in Finder and bootcamp, which it isn't. Secondly, when I tried to create a windows partition on it in bootcamp, it would only let me partition around 56GB of the 500GB total to Windows, even though the drive is totally empty.

I have a feeling this might have something to do with me previously using it as a Time Machine backup, or because I haven't used it since upgrading to Mavericks, but whatever it is I can't find a solution to it. I would really appreciate some help with this!

Cheers