I have searched around here and with Google, and none of the instructions I have found work with Oneric.
How do install exFAT and mount an exFAT partition in the latest release of Kubuntu?
You will need the package fuse-exfat to allow you to read and write on exFAT drives:
Open a terminal ctrl+alt+t and type the following:
Afterwards you just need to plug in the HDD and you're good to go.
More on this at this stackoverflow question.