Here are the required steps as provided by Kyle Kerst (SmarterTools Tech Support)
1. Choose domain to migrate.
2. Copy this domain's data, which by default is found under C:\SmarterMail\Domains\domainName, to the new hard drive.
3. When the data completes copying, stop the SmarterMail service.
4. Update the domainlist.xml to point to the new location for this domain.
5. Start the SmarterMail service.
The order is important (change to xml while service is stopped)...