Q: OS X Files starting with ._ Can someone please explain, why OS X creates ._ files?
When i copy files into another folder, everything works fine as usual, but in addition also files starting with ._ are created. For example:
I copy movie.mp4 into another empty folder, i get two files after copying finished:
._movie.mp4 (size approx.: 4kb)
movie.mp4 (same size as the original movie.mp4)

Is there a way to prevent OS X from doing so?
Because i am working with some hidden files, it is important to see them. But theses ._ files are disturbing.

A: The ._[filename] files are the extended attributes of the given file. They are usually created when copying from an HFS format disk to a non-HFS disk (like FAT)
You can recursively merge them back into the files (and delete them) using the dot_clean utility on OS 10.5+
Suppose we have these 4 .PDF files on a HFS+ volume (and the .DS_Store file):
$ ls -la
total 18672
drwxr-xr-x@  7 andrew  staff      238 Sep 17 17:00 .
drwxr-xr-x@ 10 andrew  staff      340 Sep 17 16:51 ..
-rw-r--r--@  1 andrew  staff     6148 Sep 17 17:00 .DS_Store
-rwxr-xr-x@  1 andrew  staff  2321922 Mar  1  2010 P01.PDF
-rwxr-xr-x@  1 andrew  staff  2200305 Mar  1  2010 P02.PDF
-rwxr-xr-x@  1 andrew  staff  2416249 Mar  1  2010 P03.PDF
-rwxr-xr-x@  1 andrew  staff  2608839 Mar  1  2010 P04.PDF

Now copy (with drag and drop) those same files to a FAT32 thumb drive and do a ls on that:
$ ls -la
total 18976
drwxrwxrwx@ 1 andrew  staff    16384 Sep 17 17:00 .
drwxrwxrwx@ 1 andrew  staff    16384 Sep 17 17:00 ..
-rwxrwxrwx@ 1 andrew  staff     6148 Sep 17 17:00 .DS_Store
-rwxrwxrwx  1 andrew  staff     4096 Sep 17 17:00 ._.DS_Store
-rwxrwxrwx  1 andrew  staff     4096 Sep 17 17:00 ._P01.PDF
-rwxrwxrwx  1 andrew  staff     4096 Sep 17 17:00 ._P02.PDF
-rwxrwxrwx  1 andrew  staff     4096 Sep 17 17:00 ._P03.PDF
-rwxrwxrwx  1 andrew  staff     4096 Sep 17 17:00 ._P04.PDF
-rwxrwxrwx@ 1 andrew  staff  2321922 Mar  1  2010 P01.PDF
-rwxrwxrwx@ 1 andrew  staff  2200305 Mar  1  2010 P02.PDF
-rwxrwxrwx@ 1 andrew  staff  2416249 Mar  1  2010 P03.PDF
-rwxrwxrwx@ 1 andrew  staff  2608839 Mar  1  2010 P04.PDF

You can run dot_clean to merge the attributes back into the given files:
$ dot_clean .
$ ls -la
total 18816
drwxrwxrwx@ 1 andrew  staff    16384 Sep 17 17:04 .
drwxrwxrwx@ 1 andrew  staff    16384 Sep 17 17:00 ..
-rwxrwxrwx  1 andrew  staff     6148 Sep 17 17:00 .DS_Store
-rwxrwxrwx  1 andrew  staff  2321922 Mar  1  2010 P01.PDF
-rwxrwxrwx  1 andrew  staff  2200305 Mar  1  2010 P02.PDF
-rwxrwxrwx  1 andrew  staff  2416249 Mar  1  2010 P03.PDF
-rwxrwxrwx  1 andrew  staff  2608839 Mar  1  2010 P04.PDF

You need read / run access to the directories and the files to do this obviously. 

A: This solved my problem:

  
*
  
*Open Terminal.
  
*Execute this command:
  
  
  defaults write com.apple.desktopservices DSDontWriteNetworkStores true

  
*Either restart the computer or log out and back in to the user account.`
  

