This command:
dd if=/dev/rdsk/c5t6d0 bs=4096 of=/dev/null
tries to read the entire disk and copy it to /dev/null. It may do nothing for the broken disk. But if it works, it usually causes some activity indication such as the green light flashing. You should draw a little picture of your JBOD cabinet and run the command for each disk to map them out. If you ID all of the good disks, the one that is left must be the bad disk. Once you ID a disk, use your interrupt character, usually cntl-c to kill the dd.