These spurious disconnections would result in xhci (USB3) flakiness, which would cause bus resets, which would cause write errors, which would cause occasional kernel backtraces or even panics. When a disk was failing (and several disks did in fact fail in the last couple of years, and the filesystem as a whole continued performing admirably), this was the most noticeable symptom. There was no nice, clean red light on the enclosure saying "please replace this disk". Just a whole bunch of random flakiness followed by an episode of Glyph Lefkowitz, Block Storage Detective where I had to figure out which disk was screwing everything up.