I note that NetBSD and OpenBSD (now) have their own partition types,
distinct from the old 386BSD one still used by FreeBSD.

I note that FreeBSD (now) has per-partition disk labels, requiring
relative offsets within partition, while NetBSD and OpenBSD have
per-disk labels, requiring absolute offsets within disk.

Assuming I put the right values into the relevant disk labels, is
there some way that <x>BSD can read and/or read-write ffs partitions
created by <y>BSD for <x> != <y>.  (The partitions are all on the same
disk, so I'm looking at mount -t ufs ...)

Which combinations can read?  Which combinations can write?

Are the on-disk structures similar, or have they diverged in
subtle/dangerous ways?

Any caveats?

-- 
Christopher Vance