Assuming you use meta, use metastat -p or metastat -c to see the mirror/submirror information. Then use metadetach to break the mirrors. The format of the command goes like: metadetach mirror_name submirror_to_break

Make sure that you break all mirrors from the same harddisk!!!

Regarding the new hdd, is it going to be the same as an existing hdd? If it is, you can use a simple method to initialize the disk. Just run

prtvtoc /dev/rdsk/cXtXdXs2 | fmthard -s - /dev/rdsk/cYtYdYs2

X is the source disk and Y is the newly installed disk. You can then recreate the mirrors with metattach.