Help/advice needed to add (and future-proof) second HDD to Plex/NAS server setup

Hi there,

I recently took the plunge and built my own home server, primarily to make my MacBook portable again (it was ruing Plex Media Server, and so was chained to the wall/etheet port), and although it's going reasonably well, I need a bit of help finishing it off properly.

TL;DR first:

Current setup

250GB SSD ruing Ubuntu 16.04 and PMS

3TB WD MyBook Live attached via router (etheet) hosting all media files

What I'd like to do

250GB SSD ruing Ubuntu 16.04 and PMS

4TB WD Red (+ more later on)

The director's cut:

For a number of reasons (large file transfers failing regularly, Sonarr issues) I'd like to take the MyBook Live out of the equation and move all storage inteally, starting with a 4TB WD Red and slotting in further drives later on as needed, so that all the drives are seen as one large storage area. Through putting this all together I've become fairly competent with the terminology, Terminal commands, etc, but after a bit of research I'm struggling to get the information I need to be confident I'm doing this right. I'd be grateful if anyone could help me by explaining if/how I can:

1. Add a second HDD to my box to store all my media, replacing the exteal MyBook Live
2. Set it up so I can expand this storage by adding more drives further down the line as my collection grows

It would be useful (but not essential) to be able to access the storage from other machines on the network too, for general storage/backup purposes.

Thanks