Mac Geekery

Answer

Those are Apple-Double encoded resource forks and Finder metadata for the file, such as its HFS type and creator codes and other information. Resource forks do a lot of things on a Mac and are still rather important in a lot of ways, and the dot-underscore files on DOS/UFS volumes (like a memory device or non-Apple MP3 player) are an encoded version of that.

So you can’t tell Mac OS X to not do it, sadly, as it’s a core part of the Carbon file-access APIs (so claims Apple). You can, however, periodically remove them if you’re amenable to running a shell command.

The shell utility find can also nix files it finds, so do something like this:

$ find /Volumes/YourMP3Player/ -name '._*' -delete

That should get rid of the files on the device and allow for proper operation.

You may be thinking “Well, let’s just run that on this other volume as well..” but you would be an idiot. Do not do that. This data is important for non-HFS disk formats and lets them store Mac files properly. If you remove it without understanding what you’re doing you’re liable to destroy data you actually wanted. You can do this to an MP3 player or folder of MP3s or JPEGs or similar data-fork-only formats, but very little else.