How are you trying to copy your music files to the SD card? Is it installed in the Hiby? If so remove it and put it in an SD card reader to copy the files before inserting in the Hiby.
SD card formatting
I have a 128gb SD card (SanDisk Ultra) which I formatted for a Hiby R3 Pro Saber (DAP). I chose exFat with a 128K Allocation Unit Size. My music files are Mp3 and FLAC. Is 128K (and exFat) the preferred configuration, or should I re-format to another? What would offer the best (musical?) performance? I just joined this website, so my apologies if I chose the incorrect discussion-group. Thank you for your time.