Technically, the Plex data are not ‘program’ files.
If your folder S:\Program Files has similar restrictions like the folder C:\Program Files, it won’t work.
But I’m assuming that is not the case here.
But just to be 100% sure, verify that your currently used Windows user account has full access permissions (read, write, create, delete) to the folder S:\Program Files\Plex
Install Plex server in the regular way.
You might even try and create a (small!) library to test functionality.
If all is working, go to
Settings - Server - General - ‘Show Advanced’
and put into the field ‘The path where local application data is stored’
the path S:\Program Files\Plex
Save, log out of your Windows user account, then reboot Windows.
Careful! You must not put
S:\Program Files\Plex\Plex Media Serverinto it!
