HTC One (m7ul) not found by ADB + formatted filesystem = bricked?
Woah! I'm not entirely sure what happened, but it suddenly started working again - I now have CM 11 up and running.

Here are some notes on what I was doing which will hopefully be helpful for someone else who has the same problems I did:


First off, I was running Windows 8 in a Parallels VM. I don't know if either Windows 8 or the VM was the culprit, but either way, it was silly and probably didn't help things. Getting ADB and FastBoot up and running in OS X seemed to make a difference.
Also, there is a chance that there were some problems with directory partitioning/formatting - it suddenly started working after I wiped the system a few times with TWRP (Wipe > Format Data) and rebooted. Obviously you'll lose everything, but better to lose a few files and get your phone back!


Other info that I found that wasn't relevant to me but might make a difference:


Apparently USB 3 can cause issues - try different USB ports, particularly USB 2 over USB 3.
Windows 8 may not play nice - try Windows 7 or XP (or Linux!)