A third would be to mount the drives into subfolders of the primary drive. Albeit the cheapest, it is the least preferable solution, because then Plex cannot distinguish if a missing(/temporarily disconnected) drive is only temporarily unavailable and simply assumes that the folder has been emptied out.
Thus it would remove all items from the Plex libraries, including their metadata.
And U sure that you need a drive letter for every single network drive? You do not have to give it a letter in order to access it. Letters are just making it a bit easier but they are not needed at all and even if you want to access all the drives with letters you can still collect multiple drives under one letter, like X:\drive1 X:\drive2 etc.
But i think i must give a drice letter to each one yah ?
Because when u add a new library in plex server it can only see the harddrive inside the PC or the drives that i mounted from the network. So i dont knlw how to make plex add a library by going inside a home network without mounting hummmm