The VideoLAN Forums

You most likely need to explicitly share the external drive.

Go into System Preferences, make sure "File Sharing" is turned on, and hit the "+" icon to add the external drive.

You may need to hit "Options" and enable "Share files and folders using SMB".

Ubuntu SMB server, updated to latest version (it wasn't) makes no difference. Tested on Apple TV 4k first gen, and apple TV 4th gen (both v3.3.4) Smaller files (m4v/mkv) all play ok. Not sure what the threshold is, but 3.5 gig MKV files don't play. Prolonged black screen followed by a fraction of a ...