when i copy a directory or directories of audio book mp3 files to a usb stick, thunar seems to copy them in a random order which causes them to play in random order in my car. this does not happen win i use windows explorer. is there an explaination?

I'm not completely sure, but I think thunar copies files in the order as they come on the HDD physically and not as they come alphabetically (as it is in the file manager).

Here's an example:
file order on HDD (what you can't see): File1; File5; File3; File6; File2; File4
file order in file manager (what you see): File1; File2; File3; File4; File5; File6

I suspect that thunar follows the "file order on HDD" when copying and not the other one, so this is why your files seem to have a random order (File1; File5; File3; File6; File2; File4) when you copy a whole folder.