Logging into openelec via ssh I ran parted. Once using parted I entered print all and saw that my disk was indeed there, it just wasn't being picked up. The solution was to mount the drive manually, and automate this process for every startup, since openelec doesn't adhere to fstab etc.
Within /storage/.config/ I created a file called autostart.sh. Using nano, I entered the following details (modify for your own setup)
mount -t ext4 /dev/sdb1 /media/extended
Make the autostart.sh file executable
chmod +x autostart.sh