Try this...
http://www.tleone.com/windows-mobile...rage-card.html
Google is your friend!
ps. you may want to back up your e-mail first...just in case.
pps. you will want to read through the end-user comments on that post as there is mention that if your OS tries to launch Messaging on startup, and before your OS sees your external card, it may move the messaging files back to internal storage. It makes mention of how to resolve that though.