Here is a quick way to read directly from the running kernel from which disk the system was started:
# echo "boot_string/S" | adb /stand/vmunix /dev/kmem boot_string: boot_string: disk(0/1/1/0.0.0.0.0.0.0;0)/stand/vmunix # ioscan -H 0/1/1/0.0.0 H/W Path Class Description ================================================ 0/1/1/0.0.0 disk HP 73.4GMAS3735NC #