New second 4TB hard disk only shows up as 1.6TB in my T5500 (Win7 Pro)

Hi,

I recently bought a new Seagate NAS 4TB hard disk for my T5500. The BIOS recognise the 4TB drive and it shows up as 4TB when booting to Window. However in Win7, the Disk Management only shows the disk as 1.6TB.

I have been doing some research, currently the SATA is running at RAID only mode and I think there is a 2TB limit. I don't really need RAID, but it was set by the previous owner. If set the mode to AHCI, would that solve the 2TB limit problem? Which means I have to reinstall the whole system. Is there a way around it?

Thanks.