Can't see how much memory my partitioned USB has left

Hi all

So I got my USB up and running thanks to creating a partition via ADB shell but now ES File Explorer now says "Sorry OTG USB device format is not supported" when trying to view my USB. I can't imagine I'll be using up 32GB anytime soon but it's always nice to know what I'm left with and being able to clean up unwanted files. Is the only option to plug it into your PC and inspect it manually? I can't believe how awkward Amazon made expanding your memory...