Hold down the Command and Option keys and double click on the Finder that
you want to activate. The disk that this Finder is on will now be the
startup disk.
To make the System and Finder active on the disk that an application
resides: hold down the Option key while launching the application by double
clicking on it .
As an additional note, holding down the Command and Option keys while
double clicking on MultiFinder inside the System Folder allows you to
launch MultiFinder without restarting your Macintosh.