In the context of where you store photos, videos, etc where you want to be able to use massive cheap magnetic media you would use an external hard drive. You would still boot from your internal hard drive. If you have a Fusion drive, more than likely the OS would be on SSD. If you were unfortunate enough to have the cheapest iMac with only magnetic hard drives - it would still be less of a pain and more performant to buy a small SSD external drive as a bout/app drive and an external magnetic drive running HPFS.