There is a hack somewhere but I must forewarn you that external SD cards have a limited lifespan when written to. I used to swap out and make my external the primary and it would fail after less than a year due to all the writing to it.
I can't remember where the fix is but it might be better to instead mount a directory on your external to the place on your internal.
I use an initd script that mounts various directories for me upon boot up. This way, I limit the writes and can extend the life of the card.
I've attached a screen shot of my script as an example.
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UPDATE: I don't really want to post this, but for those that like to live on the edge, here is the post explaining how to swap your internal and external SD cards. Remember, this is NOT a good idea as it WILL reduce the life-span of your external SD card as you write to it excessively. I'm speaking from experience as I lost two, yes, two 32gb SD cards because of this. I didn't know at the time that they had a limited lifespan when written to, but I do now.
Here's the link on swapping, but I REALLY suggest you do as I stated earlier. Just mount the directories you want from your external to your internal. I ensure that what I'm putting there is not getting written to a lot of times (other than when I take pictures). For example, I store game content there (downloaded content that is), my maps for CoPilot GPS app, camera pictures, music from Amazon, etc. Yes, they write to the SD card, but usually only once or sparingly, but not continuous writing.
Read post #3 for instructions. The link below should take you straight to that post:
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Sent from my Droid RAZR under an Eclipse.