Assuming you know the device name of the disc you have expanded /dev/sdg then you can simply issue the following command to force the rereading of the disk geometry.
debian02:~ # echo 1 > /sys/class/scsi_device/0:0:6:0/device/rescan
To determine the SCSI ID from device names enter: ls -d /sys/block/sd*/device/scsi_device/* |awk -F '[/]' '{print $4,"- SCSI",$7}'
For example:
debian02:~ # ls -d /sys/block/sd*/device/scsi_device/* |awk -F '[/]' '{print $4,"- SCSI",$7}'
sda - SCSI 0:0:0:0
sdb - SCSI 0:0:1:0
sdc - SCSI 0:0:2:0
sdd - SCSI 0:0:3:0
sde - SCSI 0:0:4:0
sdf - SCSI 0:0:5:0
sdg - SCSI 0:0:6:0
sdh - SCSI 0:0:8:0
sdi - SCSI 0:0:9:0
sdj - SCSI 0:0:10:0
sdk - SCSI 0:0:11:0
sdl - SCSI 0:0:12:0
sdm - SCSI 0:0:13:0
sdn - SCSI 0:0:14:0
sdo - SCSI 0:0:15:0
debian02:~ #