Well, it's somehow managed to happen two more times, to the same drive. And I have no idea how, since both times literally nothing got written to that drive in between recoveries. Everything was being written to the next disk in the array (disk 6), and it just suddenly failed, and disks 1, 2, and 5 all showed hundreds of thousands of errors in the "Main" tab of the GUI (but neither 1 nor 2 show up as "unmountable: no file system").
Last week, every single drive passed an extended SMART test with absolutely no problems reported, so I don't think there's a physical issue with any of the drives. I've also swapped out all of the SATA cables with brand-new ones in the last month.
At this point, I'm not even losing any data, I'm just getting annoyed. Would it be prudent to go through every drive in the array, move the data off, reformat, and move the data back onto all of them?
I'm completely out of ideas otherwise, and just want to stop this monthly ritual of pulling all my shit off a "corrupt" drive and reformatting it.
Also: This only ever seems to happen when I invoke the mover while I have a large number of small files (think a couple thousand video frames extracted as photos by ffmpeg) on the cache drive. I have no idea why that would cause an issue, but it seems to be the only constant.