just a list of things you can check...
-do you have access to the drive/folder that PMS cannot see in the OS enviroment? I mean can you open it within file explorer?
-after you add a folder, did you hit the restart HTTP server button on top?
-assuming you are using window:
-try moving the folder to a drive that PMS can see. if that works, try moving it back. May be the portable drive have some kind of security measure that needs to be diable first
- you can check permission on that particular folder. trying adding "everyone" as a user in the permission list and see what happens
-disable firewall and try again
-disable UAC and try again