So I'm an idiot
And last night, half-asleep, in an attempt to mount a .img to my exFAT-formatted external drive so I could boot Ubuntu from it, I ran
dd if=foo.img of=/dev/disk1
like the Ubuntu guide here says to, except I forgot the
bs=1m argument. I realized I had messed it up, and Ctrl+C'd the running program, but not before it had written a couple mb of data to the disk. It now refuses to mount on both OS X and Windows.
My question: is there any way to recover an external drive that has been corrupted in such a fashion?