Thanks for your fast replies!
I tried the solutions you suggested, with no success
- on linux fsck.vat repaired some files, but when I connected back my kindle to calibre, same error as above;
- on windows 7 I tried to repair the fs again, and windows too repaired some files; but no changes on calibre;
- i restarted my kindle, same error;
- i reset my kindle (menu > settings > menu > reset): no use: same error again.
I forgot to mention that maybe I messed up my kindle when I connected to the PC when it was totally out of charge (I wasn't been using it for some months): maybe due to the fact it has no charge, its file system wasn't able to mount properly...
What can I do now?
Thanks in advance, bye!