Well thanks to another forum we solved it. The problem is thereis a partition table in the inner memory.
If in the terminal you write:
$ cat /proc/partitions
And the output shows sdb and sdb1 It means that there is a partition table, the solution is removing it by:
sudo dd if=/dev/sdb of=/tmp/sdb.dump bs=1024k count=1024
and then formatting the sdb in fat32 with these commands:
sudo mkfs.vfat -F32 -n "Cybook Opus" -s 8 /dev/sdb
or if the output is:
mkfs.vfat 3.0.9 (31 Jan 2010)
mkfs.vfat: Device partition expected, not making filesystem on entire device '/dev/sdb' (use -I to override)
then you add -I to the last:
sudo mkfs.vfat -F32 -n "Cybook Opus" -s 8 -I /dev/sdb
Following this the Opus works again.