Can I Use An External Hard Drive To Work As A Regular Drive For My iMac? Here's The Answer.
It is possible to do this. You have to hold down the option key after the reboot chime, and it will give you the boot menu to select what Drive to boot from. Additionally, you can go to StartUp Disk in the System Preferences and choose what hard drive to boot from.
If you choose this, you must always have the hard drive plugged into it during reboot until you change it back. for more info on this, you refer to http://osxdaily.com/2013/06/22/boot-mac-external-drive/.
While I don’t have any experience using an external drive to load the OS system for any computer, but on some forums, some people said that the OS wasn’t loading or being recognized as being available. My precaution is to test it that it works correctly before erasing the native hard drive.
It looks like the iMac has a 3.5″ hard drive bay, and one article I found said that you could also take out the optical drive and put a second SSD in its place.