[crt]

Toying more with it, I found out that when there is no card, the device can 'see' the internal sdcard contents and the external empty one. It can also read files and books.
When I mount the external card all of a sudden, there is a glimpse of these directories and nothing can be read (both from the internal and external storage). The ES file explorer is the only one that can see and read them, but if a directory has many contents it also crashes.
I tried the card in another Android device and now that one has the same behavior. I suspected it might be a virus and checked the two cards with Malware Anti-Virus but nothing came out.
Strange...

[SOLVED]
Just deleted a couple of directories with their contents (on the external card) and everything is OK now.
It seems that FAT32 has an issue with many items, many of which have long names.