If you attach an extra harddisk to your virtual machine and you don’t want to reboot, all you need to do is rescan your scsi bus.
First determine how many scsi adapters your system has:
# ls /sys/class/scsi_host host0
Then order the rescan command to the adapter:
# echo "- - -" > /sys/class/scsi_host/host0/scan
Check your dmesg or /proc/partitions!