is there a way to store mail on an external hard drive using apple mail?
The answer by Zo219 is nice for having two copies, but my solution is to link the folders where Mail wants to store the files to the external drive and not have two copies.

Here are the folders you'll need to have located off the main drive:


~/Library/Mail
~/Library/Containers/com.apple.mail/Data/Library/ 


Since the volumes are separate, you can't hard link and instead need to soft (sum) link the folders or use Finder's alias function once you've moved these folders to your second drive.



That is one way that works, but I think he wants something more seamless like a remapping of the Mail directory (~/Library/Mail) to a live clone on the external drive.  I've a relative small SSD on my air (128GB) so had used a simple alias to relocate my Mail library to an external HDD at my desk.  I used an alternate user "mobile" so I could check email Mail with ease when I was away from the home office.  This recently broke under my current version of Mountain Lion (10.8.2) and I am currently looking at a workaround.  To my knowledge, hard-linking under recent versions OS X is not for the faint of heart, as it may create difficulties with Time Machine et al.  Let me know in the comments if my solution is confusing.  As far as your concern about behaviour when the external drive is not connected and you launch Mail, it just quietly fails with a polite error message about no access to an unmounted target.