If you were having issues adding drives to your steam library storage, be certain that you do not have Steam installed as a Flatpak package.

Flatpak is a sandboxed format for package management, which makes it ideal for applications to be Linux distribution agnostic. The sandboxed aspect, however, can be limiting when it comes to root access since it is not usually the case that in a sandboxed environment that you have root access.

Steam needs to have root access to access other drives in Linux. In my personal experience, this was a dealbreaker for the Flatpak package for Steam that I was using, and I promptly uninstalled it and replaced it with the apt package for Steam. All my problems were resolved.

This isn’t to say that Flatpak is bad… I quite like Flatpak! It is wonderful for applications like Discord, for example, or for Thunderbird, LMMS, Shotcut, etc. Flatpak packages, AppImages and Snap(craft) packages are helping to bridge the gap between Linux distributions. But a one-size-fits-all approach does not work for every application, and a major outlier is Steam.

To be fair, I don’t think the developers of Flatpak necessarily want to replace distribution specific package managers outright. I think they really only wish to provide other developers options for the sake of compatibility and interoperability.