Sometimes, especially on virtual machines, we need to add or resize a new drive without rebooting.More often than not, depending on the distribution and virtualisation platform in use, this happens automatically and we can easily notice a new drive popped up or its size has changed.
If you've added a new disk, but it hasn't showed up in `dmesg` or `fdisk -l`, then try to scan the SCSI bus (watch out, as there could be multiple, eg host0, host1 etc).
echo "- - -" > /sys/class/scsi_host/host0/scan
If you have resized a disk in the hypervisor, but the guest has not picked up this change, try to rescan it. Obviously replace "sdb" with the drive in question:
echo "1" > /sys/class/block/sdb/device/rescan