I am supposed to make a correction:
the command dd only read and copy a partition.
The real solution is:
sudo mkfs.vfat -F32 -n "Cybook Opus" -s 8 -I /dev/sdb
-I is really important due to this fact:
-I It is typical for fixed disk devices to be partitioned so, by
default, you are not permitted to create a filesystem across the
entire device. mkdosfs will complain and tell you that it
refuses to work. This is different when using MO disks. One
doesn't always need partitions on MO disks. The file system can
go directly to the whole disk. Under other OSes this is known
as the 'superfloppy' format.
This switch will force mkdosfs to work properly.