Scsiadd allows you to add or remove scsi devices without having to restart the computer. this is *not* a substitute for powering down to connect or disconnect hardware unless it's specifically designed to be hot swappable. use it to enable the external scsi drive you only use occasionally so is powered off when the machine first boots, or to rescan the bus after moving hot-swap drives around. scsiadd will also try to prevent you from doing anything to disrupt drive names that are in use. similar functionality is available by echoing text to /proc/scsi/scsi