If the main disk in your Mac is a spinning hard drive, you should probably upgrade to a solid state drive. Swapping in an SSD is the cheapest way to make your old computer feel like a brand-new Mac. But for backups, and for lesser-used internal storage in a Mac Pro or iMac, a hard drive still gives you the best value. You will pay far less per megabyte of storage.
The problem is that hard drives are noisy as well as slow. If you’re used to enjoying the silence of an SSD-based computer environment, then those whirrs, whines clicks and pops will drive you nuts. Which is why you should unmount your noisy hard drives. That way they’re still available to the apps that need them, but otherwise they’re sleeping.