Just for those of of that have had to replace a disk on a server or simply expand the storage space this include when you swap a disk if you have hot swap drives.
The main thing here is when you replace a drive it dose not show straight away or if you are swapping a drive it can have a different disk ID which is annoying.
To have the disk rescanned or detected you can run this one liner.
ls /sys/class/scsi_host/ | while read host ; do echo "- - -" > /sys/class/scsi_host/$host/scan ; done