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Time Machine and APFS: Local Backups

I was gobsmacked to see that Apple is now backing up a drive to itself.

With the internal SSD being highly reliable, this has a modicum of merit, but it does nothing to protect against the worst risks: theft or destruction or loss.

Apple’s approach deals with lazy users who do not backup their data: Apple is backing-up the startup drive data... to the startup drive! Ingenious. But I’ll deem this approach half-assed backups—when the poop stuff hits the fan, the data is GONE.

In About Time Machine local snapshots, Apple states:

Your Time Machine backup disk might not always be available, so Time Machine also stores some of its backups to your built-in startup drive and other local drives. These backups are called local snapshots.

  • A daily snapshot is saved every 24 hours, beginning from the time you start or restart your computer.
  • A weekly snapshot is saved every week.
  • Starting with macOS High Sierra, an additional snapsnot is saved before installing any macOS update.


Time Machine in macOS High Sierra stores snapshots on every APFS-formatted, all-flash storage device in your Mac or directly connected to your Mac. Time Machine in earlier macOS versions stores snapshots only on the internal startup disk of Mac notebook computers.

To make sure that you have storage space when you need it, snapshots are stored only on disks that have plenty of free space. When storage space gets low, snapshots are automatically deleted, starting with the oldest. That's why Finder and Get Info windows don't include local snapshots in their calculations of the storage space available on a disk.

Too bad it doesn’t work right, at least according to one reader, whoe found that these local snapshots filled up the entire boot drive, almost, and failed to delete them when needed. All while the Time Machine backup drive was attached and available.

These are not backups, but they have value in helping out users who have no concept of backup. Does this approach have merit? A little:

  • If the machine is stolen, the data is gone.
  • If a fire/flood/lightning strike whack the computer, the data is gone.
  • If the user deletes or overwrites a file, then there is value.

Stephen H writes:

I enjoy reading your blog, and appreciate the level of detail and rigour in your comparative tests. It really has helped me in important buying decisions.

I'd like to comment on your article about Time Machine backing up to a drive itself. Time Machine does this too on Sierra on HFS+, and possibly before that even.

It might seem half-assed, though I believe Apple's approach here is sensible. The local backups in this case aren't for backup, but for versioning. While on the move, I'm unlikely to have my external drive plugged in. But I still want Time Machine to provide the ability to well, go back in time to older versions.

When I get back and plug in the external drive, Time Machine dumps the local backup to the external drive. If I don't plug in the external drive, eventually TIme Machine pops up nagging notifications saying it hasn't backed up in XX days.

I feel this gives the best balance between Time Machine's key features: easy backup, easy versioning. That said, I can't answer for Time Machine going awry and filling up a drive, or other stories about its (un)reliability.

Personally, I use Time Machine for ease and Chronosync for scheduled clones. You can't have too many means of backing up for when restores fail.

MPG: Time Machine has a history of very serious bugs, like not backing up a volume. Accordingly.

If the above does not persuade the reader, there is nothing I can say that might. I don’t drive a car without a seat belt, counting on the exploding airbag.

I agree there is some quite limited value in terms of versioning, but it ought to be under user control, and it ought to actually work right; I am inclined to accept the report from one reader of TM filling up his internal SSD in a matter of an hour or two even with the TM backup drive on and attached. As a professional I cannot have TM filling up my SSD that I need for my own image files and such, and I carry backup drives.

Feeling good because it is simple and automatic is a rationalization that I reject as foolish. The fact is, anyone who does not backup to a separate drive is taking a huge risk “on the move”, if only for theft reasons, which I would suggest are at least 10X higher when “on the move”. (and what about sensitive data, financials and company info and such?) So I see not using a real backup drive as extremely risky relative to home/office use, unless the data/files involved have little value. Because the risks of theft or destruction are normally low on a day to day basis, but NOT when traveling—those risks rise considerably.

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