Q Before I take to the road with my MacBook Pro, I copy across current libraries and data for key apps like Contacts, iTunes, and Photos from my Mac Pro, which are too large to share using iCloud. I find it quickest to do this with the laptop in target disk mode, but this leaves their permissions in a mess. Why is that, and how can I fix it?
A This is probably the result of improving security policies in more recent releases of OS X.
When each user and group is created, they are assigned numbers, typically 201 or 501 for the first admin user, and older versions of OS X used those to identify the owner of files and folders when they were moved between systems. This allowed you to access files belonging to another user by copying them across in target disk mode, as the permissions were mapped to the user with the same number.
This appears to have become more complex and probably better protected of late, which can leave items orphaned after copying them across, forcing you to authenticate to change their permissions.
If both systems have the same primary admin user, with the same long and short usernames and passwords, you should be able to access them more easily if you copy them across a network, using file sharing.
However this may not work for items that are now more specially protected. You can use Access Control Lists (ACLs) to correct this, although they can quickly become a nightmare of confusion, and traditional Unix file systems employ a UMASK that sets default permissions in enclosed directories. You may have to continue manually changing permissions after copying.
Updated from the original, which was first published in MacUser volume 28 issue 18, 2012.