Do you have a steam library on the slow drive? Apparently steam usually puts in-progress downloads to the same drive as the game destination but if there's not enough room or maybe just at random steam will download to the wrong drive.
This is a few years old now but there's some discussion here https://steamcommunity.com/discussions/forum/1/1646544161774522444/
Not just "oh this is for redhat and I'm on Ubuntu" but what I run into all the time is that you find a perfect guide but it turns out to be for the wrong version of Ubuntu. So most of it works until you get half way through and you get an error because they've switched from initd to systemd or something. Then you are stuck, do you try to roll back what you've done so far? Try to adapt the instructions to the new system? Then you end up chasing your tail down rabbit holes of what is backwards compatible, what isn't, what can coexist and what can't, etc etc etc.
If you have been using a particular distro and are familiar with the subsystems then the new version comes out and you just have to learn about the few changes in the release but for someone new it adds a whole second layer of complexity to have to learn the whole new OS in addition to trying to blindly figure out how the old system worked, what's different in the new system and how you adapt instructions from the old one to the new one, or if you should just give up and try to find a different guide that will work.
I've submitted a few corrections before. Garmin or Strava used it for mapping runs and I quite liked it because in my area their maps of trails were actually much more complete and up to date than Google maps. For example in one nature park the current trails were shown on osm but Google showed a completely different set. I later came across a really old and faded sign in the park that showed trails that lined up with what Google showed despite them not existing any more. The new trails WERE shown on a pdf the city provided on their website but I guess they must have never been submitted to Google or something. Fortunately there must have been some dedicated OSM users in my area who were inputting updates.
Yeah. You could try making a symlink from the removable drive downloading folder to your fast drive but I don't know if that can cause any issues with removable drives, or what would happen if you ran out of space on the removable drive.
Another option would be to use the steam mover app to move games back and forth from your fast/removable drives and get rid of the steam-controlled library that's located there on the slow drive..