Had some time trying to get this to work again.
ssh'd in, did the /storage/.update and wget https://chewitt.libreelec.tv/testing/LibreE…h64-12.80.0.tar
Attaching a drive now doesn't crash the system and even shows unmounting when removed. But can't see any files on the drive. Huh.
So wandered off to get a nightly, LibreELEC-RPi5.aarch64-12.0-nightly-20240624-741f8df.img . Not sure if I can just find a tar straight away but the wget got some 2024.06.25 version anyway so that should have been fine.
Ah bugger, I'll just wait for the next stable release and format an emergence 64G as fat32 meanwhile. I'm getting too old for this (although usually I then get annoyed and read up until I remember how to do things ) .