hmm.. i seemed to notice that your sd0b started at the start of
the disk?  this is where the disklabel resides and unless you
specifically enable writing to that area with disklabel -W sd0
you'll get ro- failures like you're seeing.


but, overwriting this area will mean you'll blow away your disk
label ...

i'd suggest chaning sd0b to skip a few sectors at the start of
the disk (at least 8K).