As says, there are guides to adding NTFS support, if your drive is formatted that way. I’m not sure what level of support PiOS has.

Writing access was considered “experimental” with Kernel drivers. NTFS has been read-only for over a decade, but writing files is more complicated and needed an external driver up until recently. The best way would be to use FAT32, which, if the video files are large, but under 4GB, is probably the most flexible way of doing things, if you are going to use the drive with other systems, otherwise format it native Linux. Other notable premium features include early access, offline access, Plex Pass Perks, Plex DVR, timeline view, and more.How are you mounting it? Have you added it to the list of mounted drives or are you relying on the desktop mounting system? The latter will only work with a logged on user, AFAIK. Above all, you can limit your kids accessing media content with the parental control support of Plex for Raspberry Pi 4. Further, you should ensure that you have an antenna and digital tuner support. With Plex premium pass, stream live TV from various channels like FOX, CBS, NBC, ABC, etc. Plex server Raspberry Pi will sort and manage your library with the metadata, official artwork, description, etc. With Plex for Raspberry Pi 3, you can easily share your Plex library with anyone. Unlike Kodi, Plex keeps all your content synced across devices as long as you use the same account. In addition to this, you can use it to watch popular media content. It works as a centralized media organizer and helps you play music, video, images, and other media files. With the official support of Plex, you can turn your Raspberry Pi into a media center.

Raspberry Pi has been used for a multitude of purposes.

Raspberry pi 4 plex