Hallo, Hugo,

Du meintest am 26.02.12:

   mkfs.btrfs creates a new filesystem. The -L option sets the label
for the newly-created FS. It *cannot* be used to change the label of
an existing FS.

The safest way may be deleting this option ... it seems to work as  
expected only when I create a new FS on 1 disk/partition.

If you want to do that, use "btrfs filesystem label".

And that seems to work as I expected - fine.

Adding a device works, deleting a device works. Fine!
Now I'll try the job with my Terabyte disks.

(Yes - I have backups ...)

Viele Gruesse!
Helmut