How do I rescan drives I've expanded?

The easiest solution is to use scsitools script rescan-scsi-bus.

sudo apt-get install scsitools
sudo rescan-scsi-bus

To do it without installing a utility on 14.04:

echo '1' > /sys/class/scsi_disk/0\:0\:0\:0/device/rescan

IMPORTANT: Be sure to replace the 0:0:0:0 with the appropriate disk for your purposes.