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Question; Does Time machine takes 2 backups (each on one hard disk) or 1 back up distributed on both hard disks. (when using 2 external hard drives.)

Because if I select 2 different external hard disks in TIME MACHINE preferences, It says "Time Machine will take turn backing up to 'disk 1' and 'disk 2'"

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They are two separate backups. Time Machine will back up to whichever drive is currently connected.

I make use of this to alternate two Macs in my household between two Time Machine disks. Each drive contains a complete copy of each Mac as of the last time it was connected, as well as all past snapshots made to that drive that haven't been deleted to make space. Either one is thus useful on its own for restore purposes, even if the other is destroyed or corrupt.

If you connect two drives simultaneously, the same result occurs (each drive is a complete backup); Time Machine will alternate backing up to each.

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  • Thank you Zigg. So you mean to say in near future the content of both external hard drives (being backed up to) will be fairly similar. (if they're connected to same/one mac)?
    – Ray
    Commented Feb 7, 2014 at 18:11
  • Yep. FWIW, it appears (though I haven't confirmed) that connecting both drives simultaneously will make Time Machine alternate between them, which is probably what your "take turns" message is saying.
    – Mattie
    Commented Feb 7, 2014 at 18:38
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    Yes, @zigg, that's exactly what happens. I've confirmed it.
    – Alan Shutko
    Commented Feb 7, 2014 at 18:51
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    @alan, I know this is a late response, but leaving all your backup disks connected at all times is not to good in case of fire, or in case of some virus/malware that wipes or encrypts all attached volumes.
    – Arjan
    Commented Aug 24, 2015 at 10:18
  • Agreed. This should only be one part of a backup strategy.
    – Alan Shutko
    Commented Aug 24, 2015 at 16:36

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