I searched for info here and found links [1] that either didn't get answered or the solution didn't work in my case.
If there are docs on how to create m3u file to play .mp4 files, I missed them.
What's is the pathname that is expected on the usb drive? I am experimenting with file formats and locations for m3u files on usb drive that plugs into my Ultra. When I plug usb into windows or Linux, the m3u plays fine with e.g. vlc.
When I move the same drive to Ultra, no such luck. Each file in the playlist works just perfectly when I play them outside m3u.
I've tried playlist with just file, full path:
foo.mp4
./foo.mp4
.\foo.mp4
both inside directory and at top level:
Dirname/foo.mp4
./Dirname/foo.mp4
etc.
Does Ultra need say, c:/Dirname/foo.mp4
Rather than guess, I decided to ask.
This page here https://docs.fileformat.com/audio/m3u/ gives you a pretty good example of configuring .m3u file formats. (ignore the fact that it is for audio m3u, just look at structure).
The example provided by @atc98092 in the link you posted https://community.roku.com/t5/Solving-playback-issues/Roku-Media-Player-playlists-will-not-play-my-M... is a good example to configure the playlist m3u file.
-----
As another example regarding directories, see if this makes sense.
Assume you have USB drive with playlist.m3u file and two folders (foo1 and foo2). In foo1 you have movie1.mp4 and movie2.mp4. In foo2, you have movie3.mp4 and movie4.mp4.
Your playlist.m3u would be configured as follows.
#EXTM3U
#EXTINF:,This is Movie1.mp4
foo1/Movie1.mp4
#EXTINF:,This is Movie2.mp4
foo1/Movie2.mp4
#EXTINF:,This is Movie3.mp4
foo2/Movie3.mp4
#EXTINF:,This is Movie4.mp4
foo2/Movie4.mp4
--------
In the above format, the "This is Movie1" will be displayed as the title in your playlist for movie1.mp4. (Note the comma (,) before the Title you want displayed. Normally the track length time is placed before the comma if you want that displayed in info. The line below that is the path and actual name of your file.
If you use a media player like VLC to build the playlist.m3u file the EXTINFO will be created automatically for you (timetrack and title).
Also, if using media players to build the m3u files, make sure to use the exact name of your mp4 file. (VLC may add %20 to spaces in names of titles so you will want to remove those before playing in Roku Media Player (RMP). RMP does not seem to recognize the %20 as a space and your file will not be found. RMP will recognize spaces so just type or copy/paste exact name of your mp4 into this field.
----
Hopefully the above makes sense to you. If still having issues, post back with your actual directory structure with some sample titles.
This page here https://docs.fileformat.com/audio/m3u/ gives you a pretty good example of configuring .m3u file formats. (ignore the fact that it is for audio m3u, just look at structure).
The example provided by @atc98092 in the link you posted https://community.roku.com/t5/Solving-playback-issues/Roku-Media-Player-playlists-will-not-play-my-M... is a good example to configure the playlist m3u file.
-----
As another example regarding directories, see if this makes sense.
Assume you have USB drive with playlist.m3u file and two folders (foo1 and foo2). In foo1 you have movie1.mp4 and movie2.mp4. In foo2, you have movie3.mp4 and movie4.mp4.
Your playlist.m3u would be configured as follows.
#EXTM3U
#EXTINF:,This is Movie1.mp4
foo1/Movie1.mp4
#EXTINF:,This is Movie2.mp4
foo1/Movie2.mp4
#EXTINF:,This is Movie3.mp4
foo2/Movie3.mp4
#EXTINF:,This is Movie4.mp4
foo2/Movie4.mp4
--------
In the above format, the "This is Movie1" will be displayed as the title in your playlist for movie1.mp4. (Note the comma (,) before the Title you want displayed. Normally the track length time is placed before the comma if you want that displayed in info. The line below that is the path and actual name of your file.
If you use a media player like VLC to build the playlist.m3u file the EXTINFO will be created automatically for you (timetrack and title).
Also, if using media players to build the m3u files, make sure to use the exact name of your mp4 file. (VLC may add %20 to spaces in names of titles so you will want to remove those before playing in Roku Media Player (RMP). RMP does not seem to recognize the %20 as a space and your file will not be found. RMP will recognize spaces so just type or copy/paste exact name of your mp4 into this field.
----
Hopefully the above makes sense to you. If still having issues, post back with your actual directory structure with some sample titles.
There's also another playlist format that is a bit easier to create. It's the PLS format, and RMP recognizes it as well. Here's an example of one of mine. I picked a shorter one, but they can have hundreds of entries if desired. Also, if you're using Windows there's a free program called Playlist Creator 3 that can build M3U or PLS playlists.
[playlist]
File1=G:\Media\Movies\Ben-Hur (1959).mkv
Title1= - - Ben Hur
Length1=13347
File2=F:\Video3\Citizen Kane (1941).mkv
Title2= - - CITIZEN KANE
Length2=7167
File3=G:\Media\Movies\Gone with the Wind (1939).mkv
Title3= - - Gone with the wind
Length3=13999
File4=G:\Media\Movies\Midway (1976).mkv
Title4= - - MIDWAY
Length4=7889
File5=G:\Media\Movies\Spartacus (1960).mkv
Title5= - - Spartacus (1960)
Length5=11804
File6=G:\Media\Movies\The Godfather (1972).mkv
Title6= - -
Length6=10514
File7=G:\Media\Movies\The Godfather Part 2 (1974).mkv
Title7= - -
Length7=12020
File8=G:\Media\Movies_E\The Great Escape (1963).mkv
Title8= - - GREAT_ESCAPE
Length8=10329
File9=F:\Video3\The Magnificent Seven (1960).m2ts
Title9= - -
Length9=7686
File10=I:\Media\Movies_8\The Sound of Music (1965).mkv
Title10= - -
Length10=0
File11=G:\Media\Movies_E\The Ten Commandments (1956).m2ts
Title11= - -
Length11=13898
File12=G:\Media\Movies\Thoroughly Modern Millie (1967).mkv
Title12= - - THOROUGHLY MODERN MILLIE
Length12=9104
File13=G:\Media\Movies_E\The Wizard of Oz (1939).mkv
Title13= - - The Wizard of Oz:UCE 70th Anniversary
Length13=6108
NumberOfEntries=13
Version=2
The key for me was NOT having anything before the folder name as suggested in the accepted solution.
e.g.
#EXTM3U
#EXTINF:,This is Movie1.mp4
foo1/Movie1.mp4
where I tried ./foo1/Movie1.mp4 and other variations as can be seen in my original post.
Thanks all!