c0t0d0 - primary disk
c0t1d0 - secondary disk

Save output from metastat - it will show you what slice each sub-mirror is and save output from metadb -i (it will show where your disksuite databases are placed)

booted up normal:
1. break mirror
metadetach dX dXX (run metadetach command against all the sub-mirrors of the secondary disk
2. Clear sub-mirror
metaclear dXX (run metaclear on all the sub-mirrors you just detached)
3. fsck /dev/dsk/c0t1d0s0 <<-- or which ever slice would be / partition of disk you just removed from SDS
4. Mount secondary disk / partition on /mnt - cd /mnt/etc
vi /mnt/etc/vfstab - change all occurances of /dev/md to /dev/ - change all dXX to c0t1d0sX (this will allow the secondary disk to boot using all partitions on secondary disk)
vi /mnt/etc/system - remove all info dealing with disksuite

You don't state which OS version you are running - find your md.cf and mddb.cf files and rename them to oldmd.cf and oldmddb.cf

5. cd / ; umount /mnt
6. Check the physical disk location in format if you don't know it - it may not be set up already at the OBP for you to use to boot from
7. Still on booted system: cd /etc - vi vfstab - change all occurances of /dev/md to /dev/ - change all dXX to c0t0d0sX (get this info from the metastat output)
8. vi /etc/system - remove all info dealing with disksuite
9. move your md.cf and mddb.cf to oldmd.cf and oldmddb.cf
10. shutdown - you should now be able to boot either of those disks as a non-mirrored system - which means you can patch one and leave the other as a backup/backout. If after you run with the patches for a few days, you can simply recreate the one side of the mirror and then reboot and attach the second side.

Look at the different docs at docs.sun.com under troubleshooting for more help.