Thanks for taking a look! Yup I see that I'm still on 6.9.2, I'll do a flash backup and upgrade to 6.11.5. Disks 4 and 5 having little data is expected, I've been adding disks over time so most of the data will be in the first ones.
I use the Networks UPS Tools plugin to connect to a TrippLite UPS, I'll look into that.
No I don't have a "videos" share, but I have a "Spacy" share that contains a folder named "videos", that's where the data loss happened.
I don't like to mouse things into the server, I have a folder on my desktop where I put files to be moved then run a rsync script - I'm paranoid about mouse glitches too.
It doesn't seem like I'm going to get a satisfactory answer to this mystery. I'll update Unraid, install the Recycle Bin plugin, and just be extra careful from now on. I/O errors in the cache are a little concerning, a few months back I changed my cache drive into a pool of two, maybe that configuration wasn't set up correctly? I'll look into that.
Thanks again!