I missed your „edit“ because I was not informed that it was there, and I had already read your message. I would recommend that you put in a new reply instead of an edit when communicating next time.
I have some things to say. First: On my PC, under Properties, my external disk shows „File System: FAT32.“ I thought that was the recommended way to set it. Should I create a disk that is formatted as „vfat“ and test again?
Second: I have not yet tried the piCorePlayer experiment, though I started to try. As I said, it is a lot of work. And I do not know whether I should try the 32- or the 64-bit version, or both. I understand from your previous message that whatever I learn from that experiment is very unlikely to be helpful for you. So I will not put high priority on this unless you encourage me to do that, and if you do, please recommend 32, 64, or both.
Third: In studying about piCorePlayer, I came to learn that the 64-b version of piCorePlayer when running on the Rpi 4 does not support the USB fiq drivers. I have no idea what that means, but just in case it might be a hint for your debugging, I will pass that along.