Time Machine not showing backed-up files on external drive

I'm having trouble locating files in Time Machine back-ups. When I activate Time machine, the files backed-up from the internal drive show up as expected. But the files that are back-upped from a (second) external USB drive don't show up there. The external drive does not show up in activated Time Machine, so there is no way to navigate to the files.

However, the files are there. When I access the TM drive as root via Terminal, the files are accessible.

Any help as to why the files do not show up, and how to fix it is greatly appreciated!

I'm running Macos Catalina 10.15.3on an i5 Mac Mini

Koen

Update>

Solution 1:

In time machine backups, under options, is your external disk listed as excluded? Remove it, and time machine utility will be able to go back for files in the external drive from that point on (because it wasn't even backing up previously).