There is really only one media type that causes people to have the need to span drives. It is video. I use symlinks to bring my media in to a main data drive from the others. It is fast, simple and easy. Works great. Drive 1 here is the 1st data drive, not the OS drive. This can be replicated out with more drives (Drive 3, Drive 4, etc..). I usually leave 1 terabyte free on the Drive 1 for stuff besides media. Once media reaches 1 terabyte I start adding media to Drive 2 and so on. That way most of my services are using Drive 1.
Drive 1 (where main media folder resides)
create media shared folder and share it with samba
connect to share via pc and create folders (e.g. Movies, TV, Music, Photos, etc...)
create storage2 shared folder and share it with Samba (use default drop down when creating shared folder)
connect to storage2 via pc and create media folder in storage2 share
then create all the other media folders in the media fodler (e.g. Movies, TV, Music, Photos, etc...)
create storage3 shared folder and share it with Samba (use default drop down when creating shared folder)
connect to storage3 via a pc and create media folder in storage3 share
then create all the other media folders in the media folder (e.g. Movies, TV, Music, Photos, etc...)
Lastly, when space gets low in main media drive to bring in media from say the movies folders on the Drive 2 & Drive 3.
On Drive 1:
ln -s /media/uuid2/storage2/media/Movies2 Movies2
ln -s /media/uuid3/storage3/media/Movies3 Movies3
To bring in the TV files to Drive 1 (from Drive 2 and Drive 3)
ln -s /media/uuid2/storage2/media/TV2 TV2
ln -s /media/uuid3/storage3/media/TV3 TV3
(This creates a symlink file named Movies2. To break the symlink all you have to do is delete the Movies2 symlink file. If you do not know how to use symlinks it is a good thing to learn. Plex will recurse into Drive2 via this symlink and read files from the Movies2 folder and you don't need to add another folder in Plex.)
The 1st drive I do not use the number 1. But as I add drives I start with 2 then 3 then 4, etc...
PS- You can add symlinks with downloader plugin. Be aware that you can add a name manually for the destination and do not have to use the tree feature with the symlink TAB on the downloader plugin.
PPS- If you leave a lot of space on your main data drive you will not have to do the symlinks with many file types. Usually only video (Movies & TV). Other things like music takes little space in comparison and you can continue to add this to the main data drive. So if your main data drive is 2 Terabyts stop adding video to it once you reach say 1 Terabyte. Leaves lots of room on the main data drive to add vms, music, backups, etc...