umount: not coherent with fstab

[Luca Ferrari]

Hi all,
I'm using kubuntu 7.04 and I've got in my fstab the following lines:
UUID=2E96-B1E7  /mnt/disco1     vfat    
defaults,utf8,umask=000,users,exec,suid 0       1
# /dev/sda12
UUID=E63F-7400  /mnt/disco2     vfat    
defaults,utf8,umask=000,users,exec,suid 0       1
# /dev/sda13
UUID=771A-F188  /mnt/disco3     vfat    
defaults,utf8,umask=000,users,exec,suid 0       1
# /dev/sda14
UUID=72C2-1249  /mnt/disco4     vfat    
defaults,utf8,umask=000,users,exec,suid 0       1

When my machine boots the disks are mounted right, and I can use them 
normally, but if I try to umount one of them I've got a message:
umount /mnt/disco2
umount: /mnt/disco2 mount non coerente con fstab

(I guess the english for that is "mount not coherent with fstab"). Anyone has 
an idea about that? Moreover, how can I display the UUID for a partition? 
I've searched fsck for an option but I didn't find.

Thanks,
Luca