It is strongly recommended to first attempt the following procedure in a development environment (non production) prior to any production environments>How to split sites.
1. Make sure you have this WSS hotfix installed: http://support.microsoft.com/kb/939188/ (Note: This fix is also included in WSS3.0/MOSS SP1)2. Create a new, empty content database in your OLD web application. Let’s call it MySitesContentDB.3. Run the following command to enumerate all the site collections in your default content database:stsadm –o enumsites –url http://oldservername –databasename OldMySitesContentDB > MySites.xml4. Open the MySites.xml file that is created in Notepad or another text editor. You’ll see a <Site> entry for every site in the content database. Remove any entries that refer to sites that are not MySites. You should be left with a list of all of your MySites. Save the file.5. Run the following command to move all of your MySites from the default content database to the new content database:stsadm –o mergecontentdbs –url http://oldservername –sourcedatabasename OldMySitesContentDB -destinationdatabasename MySitesContentDB –operation 3 –filename MySites.xml6. Create a NEW web application to host your MySites.7. Remove MySitesContentDB from the list of content databases for the OLD web application.8. Add MySitesContentDB to the list of content databases for the NEW web application.9. Update the SSP so that it uses the NEW web application as the MySite web application.10. Test and make sure everything looks good.