Did you look at the whole screen? I believe there is a checkbox near the start button that enables formatting of all unmountable disks.
There sure was... thanks!
This isn't the first time unRaid has made me feel particularly illiterate
It's not entirely your fault. The main GUI layout has never to my knowledge been redone from the ground up. It's had bits and pieces added, moved around, improved, modified, etc, but never been totally reworked from scratch. There are many elements there that are recognizable from the first iterations that I used, almost 9 years ago.
Arguably, it works, so why mess with it, but then you get head scratchers like this for people who haven't been around it as long. If it really bothers you, it probably bothers others. If you have a constructive way to make it work better, PLEASE submit a feature request. Little details make a good product really great.