You were asking about how to pool space from disaparate sources.  You should look at greyhole (https://www.greyhole.net/). I've used it on my media server for years.  It will pull in any drive (or mount, I believe). You create samba shares and decide how many copies of the data to keep (1 to n). Greyhole manages the copies and uses the space evenly across drives.  Each drive is a standalone fs so losing one, doesn't affect data on the others. 

- Greg