> Unfortunately the logic of disklabel is overly complex because it 
contains
> code to update the bootcode on about 2 architectures.  Until this is all
> ripped out it is rather difficult to fix properly - ie separate the 'read
> disk label' and 'write disk label' paths completely.


Thank you for this explanation.
Yet, it raises another question: why did it work on sd1? :)

Cheers !
Vincent