I should perhaps have mentioned that I'm Windows user. Anyway, I created, on the uSD card, a folder named "Books.name."
But, as soon as I gave that name to it, the second dot vanished. However, I took some of my books into it, inserted the uSD card into the device and started it. The uSD card became recognised after processing the contents, and also the books in the folder were seen.
Then, I opened the book on the device that I'm currently reading, and scrolled some 50 pages through it - nothing happened (i. e. reading was without problem).
I shut the book and opened some (one after another, of course) of the books that I had loaded onto the uSD card - the same, reading was possible without problem. No crash, no popup, only that turning the pages seem to be a little slower.
It's like bedeviled - should it be that the device only pled for some time of recovery from the uSD card? - I had read the last days with the uSD card removed.
I guess it feared a factory reset.
Anyway, I shall keep an eye on the future behaviour.
Thanks to all for the suggestions!
