Here is the manual for the Roland BK-9:

https://static.roland.com/assets/media/pdf/BK-9_e3.pdf

On page 44, it says the device can handle Standard MIDI Files (SMF) with extensions .mid or .kar (I looked into .kar and it's an unofficial way to package lyrics with MIDI.)

The manual says not a word about .smf files. I am guessing they are mythical, having arisen in song and legend because "SMF files" could be misinterpreted to mean files with that extension.

Hey, here's a guy who thought he had the exact same problem, it turned out his .mid files were fine, he just needed to reformat his USB stick on the piano!
http://forum.pianoworld.com/ubbthreads.php/topics/3055662/re-have-mid-files-cant-play-on-keyboars-says-needs-smf.html

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Last edited by Mark Hayes; 11/14/21 10:37 AM.