Does it show up if you type 'mount' in terminal?
If it is a secondary drive, it will be probably be called sdb1 (it may be called hdb1)
You could try manually mounting the drive as follows:
mount /dev/sdb1 /mnt/temp
and the contents should be there as they will be if you navigate to /root/mnt/temp in the graphical file manager.
when you have finished (and are not in /mnt/temp) you can unmount it with:
Hope that helps.