Increase drive capacity from 2TB to 4TB

I bought a G-Raid Removable (2 x 2TB) and would like to know if I can change the size of the drives from 2TB to 4TB each. This is a USB 3.0, Firewire 800 unit. Does anyone know?

Yes you can change the drives, it will support 4TB drives.

I also have a G-RAID Removable 4Tb unit 0G03241 and would like to increase the drive size.

Can you confirm that the maximum possible capacity of the unit 0G03241 12Tb?
What are the make (HGST?) and model number of drives to buy for all drive size options? (4Tb and 6Tb?)

Any HGST Ultrastar HDD will be the supported models.