PLEASE HELP-Expanding storage on PR4100

I just purchased a PR4100 along with a 12TB disk drive to replace my plex server that was on my Nvidia Shield. I think I might used the wrong Raid while setting up the new device. I have 4 slots and am only using 1 of them for the 12TB.

I’ve recently bought a 8tb disk drive and wanted to add it also but when I put in the 8TB is said i needed to choose the raid. It also said that I would need to format all of the drive shows (Including the 12tb) which would erase all of my movies and tv shows. I’m only using the device for plex. How should I have set up the 12tb initially so all I had to do is when i wanted to increase my storage just pop in the drive and it adds to the 12tb i currently have without erasing anything.

My solution is to just get another 12tb to transfer the data off of the device. Reformat the drives the correct way and then transfer the data back onto the drive.

Can anyone give me advice on what raid I should be using going forward? I don’t want to risk losing data whenever I want to add more storage in the future.

Thanks in advance!!!