Thanks, I appreciate your help. However it does not seem to work.
I tried both with
Kindle for PC 1 (it downloaded a prc and mbp file) and 1.1 (it downloaded azw and mbp file).
When I open them with
Calibre, I get the error message, and then, it's true, the file is stored within a Calibre directory. However, the file is just copied. If I diff the Calibre-contained file and the original one, I get a binary match.
I am wondering what the mbp file is for. I tried deleting it, but I can still read it with K4PC, that after a while regenerates it. Is it just a bookmark index or something like that?
/dr