External Disk Partitions Messed Up

Hi,

I have a 2TB WD passport for Mac. I made a 1.2TB partition on it, splitting it so that there was my 800GB partition with 750GB of used storage. My 1.2TB partion was what I used to backup my computer to time machine through the network (but you can guess how that ended up). So now I don't need that huge separate partition for my backups, and therefore I deleted it. But instead of adding 1.2TB of free space to my first volume, it added 1.2TB of "used" space to my first volume, and "kept" my first volume at its original 800GB size.

So now I have a 2TB drive that is acting as an 800GB one, and I have 750GB of valuable data with nowhere to transfer it to in order to erase the drive and start fresh!

Any tips or advice?

thanks so much!!!