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I use Time Machine with two backup hard discs.

Does Time Machine create an identical copy of the data on both disks, or does it back up more fine-grained snapshots to different disks, so that I have more choices regarding the timestamp to restore to?

Let's say, with only one disk, Time Machine backs up hourly snapshots — 4:00, 5:00, 6:00, etc. Does it back up snapshots taken at 4:30, 5:30, 6:30 to the second disk?

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Time Machine will continue to perform hourly backups, rotating through your backup drives. Each drive will contain a full, independent backup of your source drive.

This provides redundancy for your backup system, if that is what you seek. In case one of your backup drives fails, you still have another. This can prove particularly useful if you keep one of those backups offsite (backing over the Internet will require special setup, however, and can prove agonizingly slow).

The ability to use multiple backup drives was introduced starting with OS X 10.8 (Mountain Lion).

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  • Will both disks contain exactly the same data? If I want to restore data, will it matter which disk I plug in?
    – Kartick Vaddadi
    Commented Jul 21, 2016 at 2:44
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    Since the backup disks are alternated, no. One will contain the latest backup, and the other the previous one.
    – user11633
    Commented Jul 21, 2016 at 3:31
  • Suppose I backed up to one disk on Monday, and the other disk on Tuesday, and there were no changes to any of my files in the interim (whether edits or creation of new files). Then would it be correct to say that the exact same set of files would be available on both disks, and the exact same snapshots for each file?
    – Kartick Vaddadi
    Commented Jul 29, 2016 at 6:07

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