Upgrading Drives in PR4100

This is all new to me so I’m sure I’m doing something wrong. Right now I have a 4 bay with 2 TB each, but I want to slowly upgrade to 6 TB each. I have the PR4100 set to spanning because I like one giant folder, how would I upgrade the bays and and does the HD HAVE to be WD or can I mix and match for best prices?

Check out the threads below, I suspect you are in for a rough time with how you set up your NAS.