If you aren’t sure what to do, click “Cancel.” If you want this new disk to be “the” backup disk (maybe you lost the old one, or maybe the old one failed), choose the “Replace” option. If you want to keep both disks connected, and have the Mac take turns backing up to one and then the other, choose “Use Both.”
Note: Use Both sounds good but unless you are going to keep both disks connected at all times you are going to get a lot of errors saying “Can’t back up to disk so-and-so” when it’s not connected.
So, how do we know it’s backing up? Well, we either look in the System Preferences/Time Machine to check, or (easier) we click the icon in the menu bar and see what’s what. Remember, Time Machine backs up every hour, so if your disk is connected, and you click the Time Machine menu bar icon, you should either see that it’s backing up right then, or “cleaning up,” or “preparing backup.” You won’t see anything unless you click the icon, so click. If Time Machine is not busy it will show you the time of the most recent backup. It had better be sometime within the last hour, because Time Machine is supposed to back up every hour.
One more thing: as mentioned above, Time Machine backs up every disk connected to your Mac. At least it does by default. If you have disks that you DON’T want Time Machine to back up, click the Options button in the Time Machine preferences, and exclude what you want to exclude,
What can go wrong
Sometimes you look at the Time Machine menu and you see the last backup was a year ago. Ooopsy! You haven’t been backing up. The drive might have failed, or it might not be connected (laptop owners, I’m talking to you), or maybe the checkbox in the Time Machine preferences, the one that says “Back Up Automatically” is unchecked.
Also, this doesn’t qualify as something “going wrong” but it’s still a thing: some day, your backup disk is going to be full. At that point, you either get a new disk (and you know how to set it up to be the new backup disk), or you let Time Machine delete older “snapshots” of your Mac. That’s no big deal– you’re still backed up. But, if you had a bunch of stuff in the trash the first day you used Time Machine, and then some time after the first backup you emptied the trash, you wouldn’t be able to go to to when the stuff was in the trash anymore. The Mac had a snapshot of that day, but eventually the system has to create space, so it throws out the old snapshots as needed.
Questions and Answers
Q: How long does a backup take? A: The initial backup will take a long time, at least a few hours. But subsequent backups– which will occur automatically on an hourly basis (this is why you keep the disk connected)– only need to backup up what’s changed since the previous backup. Most things DON’T change (fonts, system files, applications, many documents, most music and images) so these hourly backups don’t take long.
Q: Does Time Machine slow my Mac down? A: A little. A tiny bit. But not very much. If you’re doing video editing or music editing or otherwise taxing the Mac’s horsepower, use Time Machine’s menu bar icon to “skip this backup” to buy yourself an hour of non-Time Machine time. Otherwise, don’t worry about it— you will probably not notice the backup when it runs.
Q: Can I work while Time Machine is backing up? A: Yes. In fact, other than the initial backup, you will probably not even notice that Time Machine is doing anything.
Q: I thought everything was backed up to iCloud. No? A: No. Your iPhone and iPad get backed up to iCloud. But your Mac does not. Of course some of your stuff might be present in iCloud: mail, photos, contacts, calendars (among other things) but that doesn’t mean it’s “backed up” and especially it doesn’t mean you can go back to how things were a day ago, or a week ago, or a month ago. All iCloud is going to do is have a copy of your current stuff. You still need your own Time Machine backup.
Q: How do I know Time Machine is running? A: See my one-minute article about that.
Q: How do I know that Time Machine is working? A: See my one-minute article about that.
Q: How do I use Time Machine to get stuff back? A: That’s coming in another article. I can only write so fast.