I'm going to post what I just realized after moving from Outlook Express to Outlook and how my Western Digital Passport Elite (probably most external backups) handled it.
I installed Windows 7 this week, feeling confident that everything was backed up. I manually moved everything back when the install was done. When I moved my email, only early 2008 and earlier showed up, which was about the time I moved from Outlook Express to Outlook. Since the file structure is different, my WD backup did not back up anything in Outlook, including all email, folders, contacts, tasks, my ENTIRE life!
My backup plan was set to back up Outlook Express mail, it wasn't smart enough to back up my new .pst files which is what Outlook uses. You can't change a backup plan once it's set up. I could have created a new backup plan to add Outlook. It should have, but it never crossed my mind that my external backup wasn't getting my email backed up when I started using Outlook. I'm sure many people have been bitten by this.
Maybe if someone reads this, it'll save them from losing all email, contacts, tasks, folders, etc. Moral of the story - when you start using Outlook from Outlook Express, change your back up plan on your external HD to include Outlook.