Problem: I want to move my Mail files to an external hard drive, so I copy “~/Library/Mail” and “~/Library/Mail Downloads” to to the external drive, and then I make aliases of those two folders and put them in my ~/Library folder. However, Mail.app keeps saying I do not have the permission to modify the folder on the external drive (e.g., “/Volumes/HD/Mail” and “/Volumes/HD/Mail Downloads”), even though I clearly have Read & Write permissions to the entire disk and those folders.
Solution: Instead of making aliases using the Finder, you should open a terminal to make symbolic links instead. cd ~/Library ln -s “/Volumes/HD/Mail” ln -s “/Volumes/HD/Mail Downloads”
This should do it!