What did I do and how do I stop it from ever happening again?

[OttoKerner]

@boe_d

Also I definitely would like to move it to some place that I am backing up on a regular basis.

[HowTo] An extended guide on how to move the Plex data folder on Windows

By default, Plex server stores all its own data on the system drive, in the AppData path of the Windows user. You might want to change that location, if your system drive is a small SSD or you are simply having capacity issues. Moving Plex data to a dedicated drive also makes it easier to retain your Plex server data when changing machines or re-installing Windows from scratch. For larger servers/collections, I recommend a dedicated, internal SSD drive. Exit/End/Quit/Close plex, using the t…

See also PSA: Script to back up and restore Plex app data on a Windows system because for a successful backup you need to stop Plex server. Otherwise your backed up database will be defective.