Great! Many thanks for so much valuable details!
Something I'd also ever wanted to do is an fsck for the data partition because it looks like doing reset corrupts the filesystem. Now it is at my hand
I have latest firmware, so let's see if it works... ebook_msc is said to have stopped working with that....
https://www.mobileread.com/forums/sho....php?p=1874273 But maybe I get a workaround....