Not gonna argue on the improvement to dealing with missing files as it’s pretty basic in Engine right now, needs a much better workflow.
Here’s a random thing to check. If all files are showing on Engine OS desktop & a few files are missing on the actual hardware then the problem is likely to be foreign characters. For some silly reason Tiësto for example will be missing on the hardware but seen & operate normally on the laptop side. The fix is to change the file names & delete every foreign characters from folder names & filenames. The tag information is not affected by foreign characters so you can have é ã etc in the tagging but not in filenames
i think you need to change the tag name as well.
otherwise when using sync manager engine dj will create a “Tiësto” folder which come up as a red file on the players.
I don’t use sync manager, I update Serato DJ Pro & import my changes from there
Sync Manager is for exporting your library to a drive to use in an Engine device, not for importing music into the software.