error on boot:

devfsadm: mkdir failed for /dev 0x1ed: Read-only file system

I usually see this error when there is a meta device issue or after breaking a sub-mirror.

From the OK prompt boot into failsafe mode.

 {ok} boot -F failsafe 

Mount a disk to /a , copy the md.conf file

# mount -o ro /dev/dsk/c1t0d0s0 /a
# cp /a/kernel/drv/md.conf /kernel/drv/md.conf
# umount /a
# update_drv -f md
devfsadm: mkdir failed for /dev 0x1ed: Read-only file system

At this point metastat command should work.
We only care about s0.
Your output may vary depending on how you set up your mirror.

# metastat -p 
d0 -m d2 d3 1
d2 1 1 c1t0d0s0
d3 1 1 c1t1d0s0

Now we are going to mount d0

# fsck /dev/md/rdsk/d0
# mount /dev/md/dsk/d0 /a

We are going to look at the /a/etc/vfstab and /a/etc/system files
Always backup your files before editing.

Your vfstab should have the corresponding metadevice:

# cat /a/etc/vfstab
#device         device          mount           FS      fsck    mount   mount
#to mount       to fsck         point           type    pass    at boot options
#
fd      -       /dev/fd fd      -       no      -
/proc   -       /proc   proc    -       no      -
/dev/md/dsk/d1  -       -       swap    -       no      -
/dev/md/dsk/d0  /dev/md/rdsk/d0 /       ufs     1       no      -
/dev/md/dsk/d7  /dev/md/rdsk/d7 /var    ufs     1       no      -
/dev/md/dsk/d6  /dev/md/rdsk/d6 /home   ufs     2       yes     -
/dev/md/dsk/d5  /dev/md/rdsk/d5 /opt    ufs     2       yes     -

Your /etc/system should have this line uncommented out:
NOTE: in the /etc/system * (astrisk) are considered comments not # (pound signs)

# grep root /etc/system
rootdev:/pseudo/md@0:0,0,blk

Update the boot-archive and reboot

# bootadm update-archive -R /a
Creating boot_archive for /a
updating /a/platform/sun4u/boot_archive
15+0 records in
15+0 records out
# umount /a
# init 6

At this point your system should boot up.

If you broke your mirror as part of a patching exercise.

You would perform the same steps on your disk device instead of your meta device.

# mount -o ro /dev/dsk/c1t0d0s0 /a
# cp /a/kernel/drv/md.conf /kernel/drv/md.conf
# umount /a
# update_drv -f md
devfsadm: mkdir failed for /dev 0x1ed: Read-only file system
# metastat -p 
d0 -m d2 d3 1
d2 1 1 c1t0d0s0
d3 1 1 c1t1d0s0

Perform the steps on both c1t0d0s0 and c1t1d0s0

# fsck /dev/rdsk/c1t1d0s0
# mount /dev/rdsk/c1t1d0s0 /a

Your vfstab in this case would have disk devices as compared to meta devices.

Example of partial vfstab using disk devices.

Always backup your files before editing.

/dev/dsk/c1t1d0s1  -       -       swap    -       no      -
/dev/dsk/c1t1d0s0  /dev/rdsk/c1t1d0s0 /       ufs     1       no      -
/dev/dsk/c1t1d0s7  /dev/rdsk/c1t1d0s7 /var    ufs     1       no      -
/dev/dsk/c1t1d0s6  /dev/rdsk/c1t1d0s6 /home   ufs     2       yes     -
/dev/dsk/c1t1d0s5  /dev/rdsk/c1t1d0s5 /opt    ufs     2       yes     -

In this case /etc/system should have this line commented out:
NOTE: in the /etc/system * (astrisk) are considered comments not # (pound signs)

# grep root /etc/system
* rootdev:/pseudo/md@0:0,0,blk

Update the boot-archive and reboot

# bootadm update-archive -R /a
Creating boot_archive for /a
updating /a/platform/sun4u/boot_archive
15+0 records in
15+0 records out
# umount /a
# init 6