I've never tried this, but I would imagine it's fairly straightforward. Process should be something like this:
Get the SQL Management Studio Express if you don't already have it.
Stop the ePO services and back up the existing database.
Detach the existing ePO database.
Copy the .mdf and .ldf files to a temporary location on the new drive.
Reattach the database to the existing SQL instance and restart the ePO services.
Install SQL Express on the new drive. Make absolutely certain that you get the collation right: it must be SQL_Latin1_General_CP1_CI_AS
Move the .mdf and .ldf files that are on the new drive to the desired location.
Attach the database to the new SQL instance.
Change the database information in the config-auth page to point to the new database
Assuming everything is OK, you can now uninstall the previous instance ofSQL Express. (If it was installed by ePO, you may need to break the service dependencies before you can do this.)
Hopefully I haven't missed anything - if anyone who has done this procedure can spot any glaring errors or omissions, please chime in