> > > this is non trivial and ugly.
> > >
> >
> > making a partition being just raid partition shifted some sectors (64
> > probably) right should solve the problem for raid1
>
> why don't you run off and try that, and let us know your results?
tried some time ago but no access to that machine to see if it was really
64 sects.

i did like that:

/dev/sd0 and sd1:

a: size-of-drive-64 64 4.2BSD....
e: size-of-drive 0 RAID

then on raid0 partition a (from sector 0 of raid) is normal FFS
filesystem, installboot done on /dev/sd0a, then forced mirror resync.

works.