How to move Mail data to an SD card? I have MBA 128 GB, and my local Mail cache is too large to be stored on the HD. 
Now, I have the external 16 GB SD card plugged in via compact Nifty adapter. 
It's always mounted.

How can I set up the Mail app to store its data on that SD?
Mailboxes are stored in ~/Library/Mail. You can move this to your SD card then symlink the new location back to the original location.


Move ~/Library/Mail to your SD card.
Symlink the folder:

ln -s /Volumes/SD/path/to/your/Mail ~/Library/Mail