Why can’t my Snap-installed apps access external drives?

I’ve recently set up a new server using Ubuntu 20.04 LTS. When I try to access images on an external drive using the snap-installed Gimp I get errors that the files don’t exist (even if I run Gimp from CLI using the file I can see is there and that other apps can view). So I searched online, and every answer I find tells me I just have to enable access using:

  sudo snap connect gimp:removable-media

or via the Ubuntu Software permissions controls. I have done both of these, the perms look like they’re supposed to in both, but I still get the errors, and if I use File->Open it doesn’t show any of my external drives at all. Some answers suggested removing the snap-installed Gimp and installing via apt, but the same problem occurs with vlc, darktable, and other apps installed by snap. Does anyone know how to make this work? It’s really frustrating. My connections, including the supposedly required but ineffective :removable-media, are shown below:

garry@torvid:~$ sudo snap connections gimp
Interface                 Plug                  Slot                             Notes
browser-support           gimp:browser-support  :browser-support                 -
content(gnome-3-28-1804)  gimp:gnome-3-28-1804  gnome-3-28-1804:gnome-3-28-1804  -
content(gtk-3-themes)     gimp:gtk-3-themes     gtk-common-themes:gtk-3-themes   -
content(icon-themes)      gimp:icon-themes      gtk-common-themes:icon-themes    -
content(sound-themes)     gimp:sound-themes     gtk-common-themes:sound-themes   -
cups-control              gimp:cups-control     :cups-control                    manual
dbus                      -                     gimp:dbus-gimp                   -
desktop                   gimp:desktop          :desktop                         -
desktop-legacy            gimp:desktop-legacy   :desktop-legacy                  -
gsettings                 gimp:gsettings        :gsettings                       -
home                      gimp:home             :home                            -
network                   gimp:network          :network                         -
opengl                    gimp:opengl           :opengl                          -
removable-media           gimp:removable-media  :removable-media                 manual
unity7                    gimp:unity7           :unity7                          -
wayland                   gimp:wayland          :wayland                         -
x11                       gimp:x11              :x11                             -