How to stop Mac OS from adding hidden files to SD cards?

Are you putting these devices into Windows computers? If you are, the files do show up, but don't really do anything (well, they didn't for me). I'm not sure if you can make it stop, but you just have to deal with it I guess.

Unless you reformat the SD and flash drives as Mac HFS or HFS+ format, you can't. Those hidden files (hidden to the Mac user because they start with a period) are the resource fork information for each file. The Mac OS has to do it this way to prevent losing that information since it's impossible to save two data forks on a Windows/DOS formatted drive/card/etc. When you copy a file back to a Mac from such a drive, the two items are recombined as a standard Mac file.

you can't stop it but you can remove them before ejecting the card.

right before you eject it run the following command in terminal

dot_clean /Volumes/"card-volume-name"

put the name of your SD card the way it appears in Finder in the above.

Be aware that if you follow V.K's advice, you could lose information vital to the files. For instance, mac legacy TrueType fonts and Type 1 PostScript fonts have all of their data in the resource fork. If you remove the ._ files, your fonts will have nothing left of them. The resource fork also stores desktop thumbnails, Type and Creator codes, time stamp and other items.

Would this also explain whey when I burned a CD of MP3 files on my MBP, I see alot of werid looking files on the CD??

If you burned it as ISO 9660 from your Mac, then it's possible. In a case like that, you do want to get rid of those files right before you burn the disk. Say an MP3 is named dirtydancing.mp3. OS X then creates the resource fork file . _dirtydancing.mp3. All of the music data is in the main file and the . _ file holds nothing important for playing the .mp3. But the player doesn't know that. It sees another file with a .mp3 extension and tries to play it.

Hmm, the new forum software is trying to treat the . _ (without the space in between) as a command to underline the text. Had to change them. Stranger, it didn't do that above.