Why rebooted Macbook Pro with Mountain Lion prevents adding or moving files at Desktop?
The OS X Recovery Mode has a utility called resetpassword which, apart from the obvious, will also reset the permissions for your home holder.


Restart your Mac and hold down cmd ⌘-R, and keep holding them until the Apple icon appears.
Once started up, from the menu bar at the top, go Utilities > Terminal
Type in resetpassword and hit enter, and the Reset Password utility will open.





Select the user, change the password, and hit 'Save' to save the password, and then below that hit 'Reset' to fix the permissions