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I have two external harddrives that I use with Time Machine, alternating between them.

When I do a backup on one of them, unplug it and then plug in the other one, Time Machine will not use it automatically. I need to open the preferences, click "Select disk" and choose the drive from a menu. Then I unplug it, try to use the first one, same story.

How can I configure Time Machine to just use either of them as soon as they are attached?

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up vote 1 down vote accepted

Mountain Lion added this capability. You can simply add multiple backup disks and Time Machine will rotate between all of those that are attached at that time.

Mountain Lion - Multiple Backup Volumes

If Time Machine is unable to backup to one of them, it will notify you after 10 days, just as with a single disk.

If you are unable to upgrade to Mountain Lion, Tedium allows you to switch between multiple Time Machine volumes via a menu bar applet. You could then automate the switching based on environmental cues, such as the WiFi SSIDs available, using ControlPlane. There are also lots of scripts out there that provide the same functionality.

However, since all of these work-arounds are unsupported, upgrading to Mountain Lion would be the best option.

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(Not a Mac user, so cannot validate the answer.)

The article Automatically switching between different Time Machine disks describes an approach that could help solve this problem :

With some googling I found out, that the destination disk was written in the /Library/Preferences/ BackupAlias property and could be read with

defaults read /Library/Preferences/ BackupAlias

This gives you a rather long string starting with something like <00000000 014e0002 .... That's what you need for later. And for both disks.

Next I wrote two little shell scripts, looking like this:

defaults write /Library/Preferences/ BackupAlias '<00000000 014e0002 ....>'

and started them, when needed. No more half a dozen mouse-clicks and typing in that password every time, just calling that script was enough, but still not perfect. Jiayong Ou to the rescue, which made me aware of the context-and-location-aware and free application MarcoPolo. A little bit of configuring and I have now a fully automated backup solution at home and at the office. I additionally added the line

/System/Library/CoreServices/backupd.bundle/Contents/Resources/backupd-helper -auto &

to my script and a delay of 30 seconds in MarcoPolo, so that my backup started immediately when I'm online again and after the disk preference actually switched. Doesn't work always, since it's aborting a maybe running old backup, but seems not to do harm either.

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