So, after Christmas, I bought an external USB HDD dock, and the HDD to go in it. The purpose was to setup Time Machine on my iMac. Since it has a ginormous amount of storage, nothing I had would actually back it all up.
I got it, hooked it up, and it worked. Then I realized I didn’t format the disk as “secure”. Which isn’t awful, but I’d like my data encrypted. OSX Disk Util has this thing where it will encrypt the disk in the background, so I did that.
After that, every time I plugged the disk in, it would continue background encrypting. Then, after some period of time, it would eject itself, and turn off my bluetooth controller. Which would have been fine, except my keyboard and mouse are connected via bluetooth.
Eventually (months of not backing up later), I just re-formatted the HDD encrypted, instead of doing it in the background.
It’s all rock-solid now. Weird.