boot error on R5.0

[Kjell Midtseter]

When booting the boot sequence ends with:
-------------
mounting root from ufs: /dev/ad0s1a
setrootbyname failed
ffs_mount root: can't find rootvp
Root mount failed: 6
mountroot>
--------------
When I enter 'ufs:ad0a' after the prompt I get a login prompt.
When I log in as root I receive a '%' prompt, and I
can enter commands etc..
---------------------------------------------------------------
Background:
I want to update my firewall to use the new pf packet filter port. 
This requires FreeBSD 5.0 It is not practical to recompile kernel & world 
on the firewall box. So I do all the work on a more powerfull box and
transfer the new system to a disk that I then move to the firewall
where I then get the above error message.

After building and testing the new system I
- move all required files to directories under /mybsd
- build a disk image with bootstrap and partitioning
- insert my files from /mybsd into the disk image
- use dd to copy the image to the firewall disk

The only difference from FreeBSD 4.x that I can see is that my
/mybsd/dev is not populated. In 4.x I copied MAKEDEV from /dev
to /mybsd/dev and used 'sh MAKEDEV all' to populate my new
/dev 

So what is this rootvp?

Regards from Kjell