So you have this linux box and you want to add an extra harddrive without having to reboot it. The key command we’re looking for, is a way to rescan the scsi bus. This can be achived by rewriting some values in the /sys synthetic filesystem:
echo "- - -" > /sys/class/scsi_host/host#/scan
Replace the digit for the scsi host as appropiate.
If everything went well, you should get output on the console indicating it detected a new hard drive.