Earlier quoted context omitted.
My Kindle had this "bug" where my side loaded books randomly disappear. As a workaround, I have to keep it in flight mode at all times. Not a big issue since that’s what I would do anyway, but in case my Kindle would break, I wouldn’t think long to buy an alternative
This happend to my kindle to! After keeping in in flight mode for years I put it online again in order to buy a few new books from the kindle store, poof suddenly my entire library of side loaded books was gone, with progress and everything. I could see random metadata files related to the books on the drive, be books was gone. Super annoying as many of the books I didn't have locally anymore and to loose the "archiv…
I find it easier than converting to Kindle format and then copying over USB.