When attempting to create a rescue disk it immediately complains of a corrupt package. But it’s actually downloading the package in the background. That download stops at 98%. If I try again the same thing happens - it doesn’t resume the download, but just starts over and again stops at 98%. The error dialog pops up immediately, before or right when it starts the download.
Edit: I got it to try to install, with the error dialog in front, but it stopped at 49% that time.
That got me further - thanks! Now I get to Writing Files, with a Disk Write Error at 88%. I tried 3 different USB sticks, single FAT32 partition, with the same problem at the same point. So it’s probably where it starts blocking pending actual device writes. I did have to reformat the sticks myself first.
Edit: the specific message is “Rescue disk writing error.” The drives are USB A while this laptop only has thunderbolt, so I’ve tried two different adapters and a hub (USB 3.1, USB C connector to 4xA). The problem persists. The drives all format fine, so are definitely writeable by Explorer.