Large number of music files copied from Windows to SD card cannot be seen by music player and looks gone after inserting it

I have a dedicated music player device (TempoTec V6) running a version of Android 8. I've about 120 GB of music files in MP3 and FLAC format (which the device supports). I'm baffled about how to get the device to recognize (and not destroy) the SD card holding the files. I have a 256 and 512 GB SD card formatted in exFAT, copied the music files to them in various ways (never just in the root directory - directory size limitations - but variously to a single subdirectory (called cleverly 'Music') and to about 300 subdirectories of the various artists. There are ~11000 files. I've never successfully accessed all the files. The most recent time the card was recognized, but no music files were detected, and when I took the card back out and looked at it in Windows, the card was empty (after 6 hours of writing files and checking the integrity of the file system regularly). What's going on? Surely this dedicated music player is going to recognize an SD card... And I can't see how any device would just wipe the file system of a memory card.