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I have been using for some time Time Machine to backup to an external hard drive. I have just noticed todqy that even when my external hard drive is unplugged I can enter the Time Machine and go a few days back.

How is it possible? Is it a new feature introduced by Sierra?

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No - local backups were introduced with Lion. You can enable and disable them manually and they are turned on when MacOS detects a hard drive and portable Mac are separated or likely to run separately. The backups clear when you run low on space so most people never need to manage them.

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It is called Mobile Backups and it have been on Mac for a long time, in short:

Every time you delete a file, it'll be moved to /.Mobilebackups where it'll work like a Time Machine Backup, but if you're low on Disk space it'll automatically delete the old backups.

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  • If you want to be more precise, I believe the files are actually hard linked, not moved or copied.
    – Wildcard
    Commented Nov 5, 2016 at 4:57
  • @Wildcard well files are never deleted or moved on a hardrive. When you delete something, you just remove the reference to that memory location and when you move it, you just have a different folder reference that location. That is what a defragmentation does. It basically restructures the file tree in memory so that everything is more contiguous.
    – user64742
    Commented Nov 5, 2016 at 5:08
  • @TheGreatDuck, different layer of abstraction. I'm saying that I think the statement "Every time you delete a file, it'll be moved to /.Mobilebackups" is incorrect, because the hard link should be created long before you actually delete the file. And defragmentation isn't the same as hard links. Hard links work at the inode level and defrag rearranges blocks (or extents) referred to by the inode. Also, memory means something different from storage.
    – Wildcard
    Commented Nov 5, 2016 at 15:12
  • "Every time you delete a file, it'll be moved to /.Mobilebackups is incorrect". Yes, and that is true regardless of the mobile backups or not. Files are never moved when deleted. They aren't even overwritten until more data is allocated. Do you mean to say that there is always a link in mobile backups even before deletion? "memory means something different from storage" Memory location in the hard-drive. The hard-drive is still a memory space. It's just a really inefficient one for programs to run within.
    – user64742
    Commented Nov 5, 2016 at 18:52

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