Q: How do I switch over to my iMac secondary hard drive?

Hello JulietteVeen,

I gather the backup hard drive you installed on your iMac is an external hard drive.  As it is empty you have not been using it for backups and now want to use it for primary storage.

There are several articles on how to move data to another drive.

Note this article says Yosemite but it applies to Lion as well.

OS X Yosemite: Use external storage devices

There are also articles for moving specific things to another drive.

Back up your iTunes library by copying it to an external drive - Apple Support

iMovie '11: Copy or move a project to an external hard disk

If you do move your iTunes library to the external drive to use it there you will need to select it as the correct library.

Open a different iTunes Library file or create a new one - Apple Support

Please remember it is important to have backups.  If you decide to use your backup drive as a primary drive instead you will want to get another drive for backups.  You may want to check out Mac Basics: Time Machine backs up your Mac - Apple Support.

Best regards.