I’m on Linux Mint 21.3 and installed the latest Flatpak version but nothing in my USB SSD would open. I uninstalled the Flatpak version and installed the Appimage version and all is well.
I would really prefer Flatpak. Is this a known issue? Would opening up something in Flatseal change things?
Many thanks.
FlatPak is sandboxed (comes with limited access rights).
OK thanks. I’m not sure what directory or file should have access.
Appimage seems to work so I’ll probably stay with it.
Wherever you mount your USB SSD. I’ve also found AppImages easier to use, to be honest.
How are you mounting the USB drive?
Update: The darktable flatpak should work by default. I’ve tested it here with default settings and it works fine.
You can ignore everything below in this comment.
This is completely optional. If you want to disable complete host access (which is the default), then you can add additional settings to add explicit paths:
Assuming you mount your media from your desktop with default settings, it’s likely in /run/media or perhaps even /media (one may also be symlinked to another on some distributions; for example: Fedora has a symlink of /media pointing to /run/media … other distributions may vary, however).
You can either use Flatseal to add the directory or run this at a terminal (and then restart darktable if it was running):
flatpak override --filesystem=/run/media org.darktable.Darktable
Flatpak has a way to have an app select a directory and make it available to the app, but it needs to be done using a file selection portal.
darktable does use portals for selecting images to import, but it doesn’t seem like you can use a portal to select a place for your library. The above command (perhaps with a modification on the path, depending on your distribution or if you manually chose a mount point in /etc/fstab or a disk tool) — would work around the sandbox, to make it available to darktable regardless of its own support.
It seems that filesystem=host is more limited than I thought (or has become more restricted overtime): Sandbox Permissions - Flatpak documentation
darktable’s flatpak uses the host permission by default.
Hrm… This is still annoying, yet we’re flagged by flathub as being"insecure" by using these permissions.
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.
Could Linux Mint be mucking with the default permissions?