You're running into the limitations of having a bios.
To go over 2TB on a single (virtual) disk, you have to change to a different partitioning style; you have to change to GPT (default partitioning style is MBR). However, a computer that uses a bios, cannot boot the computer to such a disk.
You have a few options:
- create 2 virtual disks on the same set of physical drives; one for boot, the other for the rest of the disk space (downside is that you'll never be able to expand this raid set to include additional drives due to having 2 virtual disks on there)
But yes you should be able to use the extra drive as another volume no problem at all.