cksum will just read through the entire file, doing a checksum. If that command gives you a “permission denied” you’ll know that the grafana-server process can’t read it either.
There’s a lot of backtracing to do to fix the permissions. I think I would do this:
sudo -u grafana bash
That way I’d have a bash shell that is running as the grafana user. Then I would try listing everything at each level of your absolute path to make sure I could read the files at that level, something like this:
ls -l /media
ls -l /media/gb
ls -l /media/gb/DATA
blah blah blah
Gotcha, no problem. You see the line where it shows gb says drwxr-x--- and then root root. What that means is that it’s a directory (d), it’s owned by user root in the group root, and that the user root has read-write-execute (rwx) and others in the group root have read-execute (r-x). (For a directory, x means you can “enter” the directory.)
Because you are grafana, you cannot enter that directory or read anything in it. I would recommend something like this (if it’s okay for every account to be able to enter that gb directory):
chmod o+rx /media/gb
That should change the permissions on that directory so that every user can see and enter it.
There’s a good amount to know about Linux file permissions, here’s a basic article about it: