According to /dev/MAKEDEV we no longer make

crw-r-----  1 root   operator  24,      2 Oct 10 17:12 rmd0c
crw-r-----  1 root   operator  24,      1 Oct 10 17:12 rmd0b
crw-r-----  1 root   operator  24,      0 Oct 10 17:12 rmd0a
brw-r-----  1 root   operator  17,      1 Oct 10 17:12 md0b
brw-r-----  1 root   operator  17,      0 Oct 10 17:12 md0a

ie., no md0a, just md0, md0c. However I think that in src/sys/kern/kern_subr.c
we still look to mount /dev/md0a rather than /dev/md0 as the rootdevice.
I am not sure where though! (Probably in something with 'a' + bootdevice
where bootdevice=0)

Clue: boot up with an INSTALL kernel and spot the line

root on md0a dumps on md0b

Cheers,

Patrick