My lessons learned: Format mobile discs with Mac OS extended instead. Backup continuously on a different drive (also formatted with Mac OS extended). I'm happy with ChronoSync, btw. Run 2 mobile discs at most on the USB hub. Before I add any other device to the hub, an USB stick for instance, I unmount the backup disc. Just to be safe. Hope this helps.

Welcome to the club of a problem that nobody at Apple or any 3d party seems to bother to find any other solution than formatting the disc. My external USB discs got damaged when I attached too many of them (actually 3) to an USB hub. Which seemingly produces a power spike that the system reacted to with throwing out a disk without unmounting it properly. Which is a particular bad thing with USB discs that are formatted with APFS, as we had to find out.

My "last hope" is connecting the corrupted drive to a linux machine and try out what was suggestd. Until then I'm not reformatting the drive.

Incidentally i bought an iPad 10th Gen with an USB C port. It seems that USB provides enough juice to power the USB C drive. BUT: where do the files on the corrupted drive become visible on the iPad???

Funny little trick doesn't work with an iPad with lightning port as I've just tried it out. If I ever get my hands on an iPad with USB c port I'll report if I can reproduce makafarai's result