I have a client that I'm setting up Time Machine to backup his Macbook Pro. He's running OSX Mountain Lion, but soon to upgrade to Mavericks.

It has been backing up using Time Machine and an external USB drive.

He wants to add a 2nd USB drive to get a weekly backup that he'll store offsite.

We've setup the 2nd drive and had Time Machine point to that 2nd drive and it is creating an initial backup now to it.

My questions:

  1. If he unplugs the 2nd drive after the initial backup and then takes it home, when he plugs it back in next week does it backup what has been changed since today's initial backup to it? Meaning, does Time Machine base the backup off of the destination media in conjunction with the source? Or will it only backup what hasn't been backed up in the last hour (assuming the 1st external drive is plugged in this whole week and Time Machine has been backing up as normal to it)?
  2. Will Time Machine treat each external backup drive as 2 independent backups, each with their own "sync" with the source hard disk?

Basically, what I want to make sure happens is:

  • EXTERNAL DRIVE #1 = NORMAL backups....daily/hourly/weekly/etc. always plugged in.
  • EXTERNAL DRIVE #2 = WEEKLY/DR backups....only plugged in once a week (or less) and backs up everything since the last time it was plugged in...regardless of what the backups for EXTERNAL DRIVE #1 have been doing.

Thank you.

2 Answers 2


Basically, I would recommend not trying to constrain time machine to only a weekly interval on the offsite drive.

That being said, you can have both drives work totally independently as that is how the software is engineered. If you really only want a weekly snapshot, only connect the weekly drive once a week and force a rotation of the destination:

 tmutil startbackup --rotation

Once you have a backup to the weekly drive, eject it and send it away. As long as all dives get reattached every 14 days or so, you won't get a warning dialog that any are out of date.

Again, you can certainly enforce a single weekly backup by the procedure you follow but two things make that practice less needed than you might expect:

  • The backup intervals are very low space overhead compared to the size of cheap backup disks.

On my worst Mac, the system averages 5 MB of overhead for system log files so in absence of user files changing, you have 24 daily and 31 monthly snapshots you would avoid storing 275 MB (or 0.0003 of a one TB drive) if you were on a strict one per week schedule.

Since the cleanup of old backups is so fast and the storage impact so low, most clients with offsite rotation end up just swapping disks each week and let the software cull the extra snapshots per the normal schedule, leaving weekly snapshots more than a month old.

  • OK, thanks. So they obviously don't work as Windows files with archive bits then? More like an rsync backup where it compares source/destination?
    – TheCleaner
    Commented Dec 27, 2013 at 14:12
  • Exactly, there is a filesystem events daemon that can track all changes to the source filesystems and rapidly know what files were touched as well as a rsync like deep scan fallback so that multiple destinations can be sure that a new backup interval correctly captures all changes since the last backup to that specific device.
    – bmike
    Commented Dec 27, 2013 at 14:14

I didn't get it. Does Time Machine copy 2 backups on the 2 external hard disks or uses 2 hard disks to take one backup.

I set one external hard disk as the backup destination. Then again opened Time Machine Preferences and selected another one. The message appears as " if you use both hard disks, Time machine will take turn backing up to ... and ..." Does it mean one back up or two back up?

  • 1
    Welcome to Ask Different! This is really a comment, not an answer. With a bit more rep, you will be able to post comments. For the moment I've added the comment for you, and I'm flagging this post for deletion.
    – TheCleaner
    Commented Feb 7, 2014 at 16:59
  • 1
    This would make a good stand alone question as well.
    – bmike
    Commented Feb 7, 2014 at 17:36
  • Time machine alternates destinations so you have one backup per hour and half as many snapshots on each drive as if you only had one destination.
    – bmike
    Commented Feb 7, 2014 at 17:38

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .