Seagate 3TB USB 2.0 External Hard Drive

Hello,
I bought a Seagate 3TB 2.0 external hard drive. Initially it would not work on my PC but with some help from the Seagate website I got my PC to recognise it but the HDD has been split into two partitions - a 2 TB one and a 800 gig one.

The 2TB works fine but I can't access the 800 gig - I go into Disk Management & if you right click on that volume, it gives you no options.

I was told it could be because of my motherboard not reecognising anything over 2TB & to get a BIOS upgrade but it seems one does not exist. The motherboard is Gigabyte P55-UD3.

Does anyone know how to get access to that other 800 gig because otherwise I am stuck with a 2 TB HDD but paid for 3 TB?

I am also using Windows 7 if that is needed.