Time Machine is designed to back up your selected volumes every hour, writing any changes you’ve made into a new snapshot that captures only the differences between the previous backup. (Apple software, like Pages and Numbers, and a lot of third-party software create a version every time you save, providing backups within apps much more frequently than hourly.)
But some readers wonder what happens if they don’t keep their Time Machine drives powered up all the time. Apple designed Time Machine to be resilient, so it doesn’t skip a beat if one or more of the destination volumes you use for backups isn’t always available.
The ideal situation is that any drive you use is always plugged in and thus backups always happen while it’s on. This is particularly useful—maybe critical—if your Mac acts as a networked Time Machine destination for other computers on the network, as they may be backing up at times you’re not using your Mac—if a partner is working late or one of your kids is writing their term paper at 2 a.m.
