Apple's Time Machine works in a funky way, so go over to your local store and get a large 4TB (or bigger) drive. Also, get Shirt Pocket's SuperDuper program, which will make a bootable external disk that replicates the internal drive in your system. In case something goes wrong with your internal drive, reboot your Mac while holding down the "option" key, and boot the backup. The newest version 3.2.5 supports Mojave and includes their Smart Delete, Smart Wake, and Smart Update features. In addition, look into NAS (Network-attached Storage) drives if you have several critical media files that can't afford to get lost. Cloud storage via Google Photos, Amazon Prime, Shutterfly, etc. is also an extra layer of backup for peace of mind.