I would suggest to try the dot_clean command for now to get read of those ._ files on your Sansa. I may be wrong on this, but this is what I have observed after using my freinds MAC computer. It seems that those ._files are only generated after transferring a converted audio to mp3 to the Sansa. You can easily get rid of those ghost files by running the dot_clean command under terminal.
Launch Terminal (Applications > Utilities) and type dot_clean, making sure to include a space at the end and drag the icon of the Sansa to the terminal window and press on enter. Disconnect the Sasna and those ._ files should be gone. :smileyvery-happy:
