Does the drive not show up in Baobab, the GNOME storage manager?
I do not know what BackBlaze is but could investigate. I also don't know why Steam can't find your HDD... I wonder if there's a visibility issue at work there.
Clicking the options in the bottom-right speaker icon should let you switch between different output options. I think you can also type "sound" in the menu and find them that way. Do you mean it's silent for at least one of the outputs?
Which games are failing? I presume they're listed on ProtonDB, right?