Alright, first things first. Let’s isolate the issue here. Since you’ve already made sure the disk is mounted and permissions are set correctly (or so it seems), AMP might still be unable to write to the datastore properly. Permissions can be a bit finicky with mounted drives, even if they look correct on the surface.
Let’s see if the amp user actually has proper write access to the mounted drive. Try switching to the amp user first by running:
sudo su -l amp
Once there, navigate to the /mnt/amp/instances/ folder and attempt to create a simple file:
touch testfile && ls -l testfile
If that fails, permissions or ownership need re-checking. Let me know what happens!