Thanks. I've examined the differences between my BATOCERA partition, and the files in the ZIP file, and the only difference seems to be the contents of recalbox.conf. The file batocera.update shows as batocera on my USB, but a diff suggests the contents are the same.
I don't think disk space isn't an issue. The BATOCERA partition uses 675MB of 1.9GB, and the SHARE partition 2.8GB of 13GB.
I wiped the BATOCERA partition and extracted the files from the ZIP, but the boot process stalls at the same point. I tried wiping the USB entirely and flashing it again, but the 5.19-x86 version won't boot at all on my 64-bit PC. It's fine on the 32-bit laptops, however.
The previous version 5.18-x86 DOES work on my 64-bit PC, but only if the first boot is done on one of my 32-bit machines. :-/
I've no idea whether that's just a coincidence or if there is actually a problem running Batocera x86 on 64-bit machines...?