How to fix: mv: failed to preserve ownership

mv is the wrong tool for this job; you want cp and then rm. Since you're moving the file to another filesystem this is exactly what mv is doing behind the scenes anyway, except that mv is also trying to preserve file permission bits and owner/group information. This is because mv would preserve that information if it were moving a file within the same filesystem and mv tries to behave the same way in both situations. Since you don't care about the preservation of file permission bits and owner/group information, don't use that tool. Use cp --no-preserve=mode and rm instead.

But if you don't care about the warning, mv actually does move the files before complaining ownership problem.