I'm hoping everything is okay. I'll attempt to backup some more data before rebooting. Just in case.
Update: Proof that data storage is cheap these days - it's not worth stopping the data copy to delete my cache. Once upon a time I never would have backed up my web browser caches, even by accident, since they were too space-consuming.
Update #2: Well, my data is all backed up, and the computer seems to be working fine, to my great relief. I'm still expecting to find something buggy somewhere, with the updating software if nothing else, but if I'm lucky, it'll be nothing serious.