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

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???

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.

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 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.