First I would recommend to add the HDD to /etc/fstab for mounting on boot.
Then please read the man page:
man minidlna.conf
There you see:
media_dir
Path to the directory containing the media files minidlna should share.
Use this option multile times if you have more than one directory to share.
Example:
media_dir=/opt/multimedia/videos
media_dir=/opt/multimedia/movies
You can also restrict an entry to a specific media type, you do this by using the following syntax:
the letter 'A', 'V' or 'P', followed by a comma (',') followed by the path.
The meaning of the first letter is as follows:
'A' for audio files
'V' for video files
'P' for image files
For example, if you want to include only video files located
in /opt/multimedia/videos directory, and only music
in /opt/multimedia/music, then you would use
media_dir=V,/opt/multimedia/videos
media_dir=A,/opt/multimedia/music
Another example would be
media_dir=V,/opt/multimedia/videos
media_dir=V,/opt/multimedia/movies
media_dir=A,/opt/multimedia/music
Or, if you did not care what type it finds, then you could use
media_dir=/opt/multimedia/videos
media_dir=/opt/multimedia/movies
media_dir=/opt/multimedia/music
You can mix it up, find anything in music, but only Videos, in videos and movies
media_dir=V,/opt/multimedia/videos
media_dir=V,/opt/multimedia/movies
media_dir=/opt/multimedia/music
That should explain it and edit the file /etc/minidlna.conf
After restarting the service, it should be applied:
sudo systemctl restart minidlna.service