copying just / (not /tmp, /usr, etc) (rsync -x failed)

[Steve Franks]

I have / on one slice, and [usr,tmp,var] on others.  I want to move
just / to a new disk, which seemed to be what rsync -x ("do not cross
filesystems") was intended for.  It failed, however, as df shows 20k
blocks in /, and rsync filled up the target slice with 50k blocks, so
obviously it blew right past the 'end' of / - did I miss something? Is
there no other way except to umount [tmp,usr,var]?

Thanks,
Steve