using a disk "mirrored" with dd

[Matt]

I have a machine running 4.10-RELEASE-p2 with two identical hard disks
-- ad0 and ad2. I would like to make a complete copy of the first disk
to the second disk nightly for quick disaster recovery.

I've dd'ed the disk as follows

# dd if=/dev/ad0 of=/dev/ad2

and edited the disklabel of the second disk to change ad0s1 to ad2s1 as follows

# disklabel -e -r ad2

However, I'm still unable to mount the root partion (or any other
partition) of the second disk to tweak /etc/fstab etc. even though

# mount /dev/ad2s1a /mnt
mount: /dev/ad2s1a: Operation not permitted

What am I missing? Is there anything else I need to do beyond editing
the disklabel to allow me to use the "mirrored" disk?

Output of disklabel for the two disks follows:

# disklabel ad0
# /dev/ad0c:
type: ESDI
disk: ad0s1
label: 
flags:
bytes/sector: 512
sectors/track: 63
tracks/cylinder: 255
sectors/cylinder: 16065
cylinders: 4864
sectors/unit: 78156162
rpm: 3600
interleave: 1
trackskew: 0
cylinderskew: 0
headswitch: 0           # milliseconds
track-to-track seek: 0  # milliseconds
drivedata: 0 

8 partitions:
#        size   offset    fstype   [fsize bsize bps/cpg]
  a:   409600        0    4.2BSD     2048 16384    97   # (Cyl.    0 - 25*)
  b:  5242880   409600      swap                        # (Cyl.   25*- 351*)
  c: 78156162        0    unused        0     0         # (Cyl.    0 - 4864*)
  e:  4194304  5652480    4.2BSD     2048 16384    89   # (Cyl.  351*- 612*)
  f: 10485760  9846784    4.2BSD     2048 16384    89   # (Cyl.  612*- 1265*)
  g: 18874368 20332544    4.2BSD     2048 16384    89   # (Cyl. 1265*- 2440*)

# disklabel ad2
# /dev/ad2c:
type: ESDI
disk: ad2s1
label: 
flags:
bytes/sector: 512
sectors/track: 63
tracks/cylinder: 255
sectors/cylinder: 16065
cylinders: 4864
sectors/unit: 78156162
rpm: 3600
interleave: 1
trackskew: 0
cylinderskew: 0
headswitch: 0           # milliseconds
track-to-track seek: 0  # milliseconds
drivedata: 0 

8 partitions:
#        size   offset    fstype   [fsize bsize bps/cpg]
  a:   409600        0    4.2BSD     2048 16384    97   # (Cyl.    0 - 25*)
  b:  5242880   409600      swap                        # (Cyl.   25*- 351*)
  c: 78156162        0    unused        0     0         # (Cyl.    0 - 4864*)
  e:  4194304  5652480    4.2BSD     2048 16384    89   # (Cyl.  351*- 612*)
  f: 10485760  9846784    4.2BSD     2048 16384    89   # (Cyl.  612*- 1265*)
  g: 18874368 20332544    4.2BSD     2048 16384    89   # (Cyl. 1265*- 2440*)

Thanks for any insight, and let me know if you need to see anything else.

-Matt