The drives get disconnected or the system crashes and the mount point gets stuck as a folder. When the drives go to mount again that folder prevents the drive from using that mount point name. One option is to safely remove (unmount) the connected drives, manually delete the folders in /media and then plug the drives back in. Optionally, and probably better for the long haul, would be to switch from the automounter to manually configuring the mount to /mnt using autofs which is much more graceful with mounts…
To fix the mount point change without much fuss you can use path substitution…