Mount error: "Specified device does not match mounted device"

[jjf at mail.cybertinker.com]

Hello to all.  Sorry if this is a little terse, but a production machine is down
and I'm very tired.

Our mailserver (running 4.5-STABLE) died today, due to massive read errros
from /dev/sd0s1a -- the root partition.

I figured we'd just replace the primary drive and all would be well.

I put the spare drive into a 4.9 box -- all I had available -- and
created my slice and partitions.  I did not match the parition sizes
exactly with previous drive, but in each case they were larger, so I
knew I'd have plenty of space.

So having made my slice and partitioned it, I pulled my backups off of
tape, transferred them to the 4.9 machine, and used 'tar xpzf' to write
them to the shiny new partitions.  All seemed well.

We put the new drive into the dead server, and fire it up.  Boot was halted
due to inability to load mount a vinum device.  But set that vinum issue
aside for now.

'/' was mounted read-only, and I attempted to force it into read-write
with 'mount -f /', which has always worked for me.  But not this time.
I got: 

  mount; /dev/ad0s1a on /: specified device does not match
  mounted device.

First thing I did was to verify that I had the proper entry for '/' 
in /etc/fstab, and indeed I did.  

I was however, able to successfully mount the other partitions I'd
created on the replacement disk.

I ran 'mount' after mounting the other partitions, and I saw a strange
thing.  All but one of the partitions were labled with their full device
name (i.e. "/dev/ad01s1f on /usr (ufs, local)"), but it was different 
for ad0s1a (the root partition).  It said "ad0s1a on / (read-only)".
What is the significance of this, the root partition, being only
partially labled  like that?

Note: this drive was ad0 on the mail server, but was ad1 on the 4.9
machine.  Could this be the cause of the problem?  Do the drives
somehow cache their most recent system designation?

I was also wondering if maybe the problem is related to disk partitioning
on a 4.9 machine and sticking the disk into a 4.5 machine.

I have seen in the archives other people with this general problem, but
it seems that in those cases the problem was incorrect entires in /etc/fstab,
so we're very confused here.

Any help would be appreciated,

John