If you have created an Additional Encrypted Drive which you want to make your Primary Encrypted Drive, you will have to load it first.
Use the "More Drives" button to open a new CryptArchiver window, and then "Load" the Additional Encrypted drive. You can now use the [File -- Make this the primary encrypted drive...] menu to set the current drive as your Primary Encrypted Drive.
For additional security and privacy, CryptArchiver does not remember the names and locations of Additional Encrypted Drives that you create. However, for your convenience, it does remember the name and location of the first drive you create, or your Primary Encrypted Drive.
When you start up CryptArchiver, it automatically tries to load the Primary Encrypted Drive. You will be prompted for the password to your Primary Encrypted Drive.
Setting your Primary Encrypted Drive allows you to conveniently load your most-used drive automatically when CryptArchiver starts.