I have a strange issue with my Plex Media Server on Windows 10. On initial setup, my Plex server sees the mapped NAS (P) drive letter. I can successfully add the folder with all my video. Plex scans the files and everything works perfectly. If I reboot Windows, Plex drops access to the P drive letter even though Windows has access to the drive. I can access the P drive in Windows with no issue but Plex no longer sees it. If I uninstall Plex and reinstall it will see the P drive again.
Yeah it has to do with drive mapping and accounts, Im gona bet you are using plex as a service, and thus it is running as system, while your account uses your ID. When you install it, it is running as you, when you reboot, it is now running as system, and looses your drive mappings.
Drive letters are restored as one of the last things after logging in to a Windows user account.
It might just be that the server has been started too early on.