You booted to a cd and are trying to change root's password? You can't just use passwd since it's trying to write to the CD and it's not writeable. If you're trying to modify root's password on the system disk (/dev/dsk/c0t0d0s0 for example), you'll need to mount slice 0 and edit the shadow file.
Code:
# mount /dev/dsk/c0t0d0s0 /a
# cd /a/etc
# vi shadow
