Can I replace one of the 4TB Drives on my GDrive 8TB?

G-Technology G-RAID & G-DOCK
1

If so, do I need to get the exact same drive? Any instructions on how to do that?

2

You posted in a G-RAID section but mention G-DRIVE in the subject.

What device do you have? And why would you replace only one of the drives if it is a RAID? they need to be identical drives.