Oh! Does host not work? I as assuming it was not on already; I’ll test again here with my RAID over USB (what I usually use) instead.
I often use either the Fedora or OBS builds of darktable, not the Flatpak, as I wanted acceleration for AMD via ROCm… which isn’t working in the latest release (it glitches out; darktable becomes unstable on both OBS and Fedora packages… I need to report the bug).
OK, darktable works by default from Flathub for me, even for my standard USB disk. 
Edit: I’ve updated my comment above too.