This is potentially a complicated discussion with 3 major options. 1) Use a cloud service like Drop Box, Google Drive, Apple's cloud service. All your data must be stored in a spot where it can be synced. This is potentially complicated to setup to be confident it is working. 2) Buy a second hard drive and use backup software to backup the first drive to the second drive. This option has an issues. 1) If you have a fire/theft you may loose both the primary and backup hard drive. 3) Use a cloud backup service. See link below for Mac Options. This will ensure you have an off site copy in case you have your hard drive stolen/lost/destroyed.

• Also See: https://www.cloudwards.net/cloud-backup-for-mac/