I have a Plex Lifetime Pass on Win10 Pro x64 box with small standalone C: (root drive) and D: 25TB RAID.
This question has probably been asked a lot but after wandering around in the FAQs and forums for an hour without finding an answer here goes. BTW, for such a nice package, the user support is weak at best
How do I make Plex put all of its files on my D: drive?
Apparently even after installing to the D: drive Plex still puts a ginormous number of files on my C: drive which has limited space. I have seen this question asked a lot of different ways but the answer is never given.
It sounds as though your Plex data directory is still being stored on your C: drive. You can specify in Plex’s settings where this data should be stored:
Settings → General → The path where local application data is stored
Note: Changing this setting doesn’t move already existing data for you. You’ll have to do that manually. This article can help you find the default location. And this article describes the setting above is a little more detail.
So, stop Plex Media Server, move the data directory to where you’d like it, change the setting above to match, and then start Plex Media Server.