How many 3Tb drives do you have installed IN this computer, by the way? This motherboard has a UEFI BIOS, yes? And you're using Win7-64?
When the Seagate Ext is plugged in, and you pull up the DISK MANAGEMENT (under Administrative Tools / Computer Services / Disk Management), did it give you the chance to format it originally using MUI (MBR) or GUI?
Or did you install some Seagate translator utility that promises to make the 3Tb drive work with any computer?
I've never used that because I am suspicious of HDD translator software that promises backward compatibility. I'm wondering if this is an example of the feared drive problems?