I can finally clear this up! It was indeed the latter. After yesterday's LOS update the animation was up for three hours or so until I stopped it by switching the phone off with a power long-press, after which it booted up normally—but without any trace of my 512 GB Kingston "Canvas Go! Plus" micro sd card. Seems a file system check was taking it so long. (On my laptop, I could read the card ok, but a file system check aborted with read errors.) Definitely one of the more expensive cards I bought, and it seems it doesn't do much (if any) wear levelling. Never happened to me before except with