How can I find what is using the rest of my drive?
I use Daisy Disk to keep tabs on what's using up space on my disks. Like @Hoshts and @Joonas have mentioned in their comments on another answer it's not free, but gorgeous and easy to understand. Money well spent, in my opinion, unless you're just looking for a one-time solution.

One thing to keep in mind - the Mac App Store version of Daisy Disk is sandboxed - which means only the files your user can read will be counted.

If you use the direct download version of the app (which will assume the license from the MAS version if you install both and run the direct version after launching the MAS version one time and quitting it.) - it can run as Administrator and show all the files for all users on your Mac.