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Ok, so rotating two disks with Time Machine is rather simple. Probably simpler than using 3rd part to do backups (from this worth reading FAQ about TM) of it.

I actually like to keep half my macbook's disk for TM and I want to add an external disk.

Problem is: I can only choose one to backup on TM's settings.

One possible solution is setting up TM to a RAID, but I'm not sure this is a good idea, and I believe it would require both disks (or partitions) being the same size.

Currently I think it's better to just manually rotate, by going to settings and changing the selected disk daily, or whatever frequency.

I think the question here is: how can I do this automatically?

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Have you considered instead to simply synchronize your MacBook TM to the external on a periodic basis? Using a proper tool like SuperDuper! you would create an exact copy of your Time Machine backup on the external, and it would be recognized by Time Machine's restore function in the event of an HD failure. I'm not sure if Carbon Copy Cloner fully supports copying Time Machine backups, but SuperDuper! definitely does.

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Yes, I have considered, just figured from my link and other places it would not be a good idea. Are you sure TM will be able to recognize it? Maybe I could even use Disk Utility? Either way, I'd still be missing a way to automate it, but I could try automator (I've never really used it). I'll try this later anyway. Thanks! – cregox Feb 17 '10 at 7:23
Carbon Copy Cloner and Disk Utility didn't work, as expected. Trying SuperDuper now, it should be fine. But I think some script to auto-rotate Time Machine would still be generally better. – cregox Feb 18 '10 at 19:40
up vote 1 down vote accepted

Found it!

That is the kind of thing I was looking for!

I'll just put an adapted quote from the author (pbeyersdorf):

The script is launched automatically when my external drive is mounted or unmounted (using the free utility Do Something When) and uses GUI Scripting to set Time Machine to backup to a "primary drive" (the primarydrivename variable in the script) if it is available, and if it isn't, it will attempt to have it backup to a "secondary drive" (secondarydrivename). Failing that, it leaves the Time Machine preferences unchanged.

Just be sure to read the first link as the script, made before Snow Leopard, must be modified to work with it!

I'm just going to use it with timer rather than DSW!

Happy! :)

This is much better than Super Duper, which seems to take a lot longer Time Machine, or Disk Utility, which can't make progressive and might only be worth it for the first time setting up the second TM disk. CCC doesn't even work with TM.

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Good to hear that it works! – fideli Feb 19 '10 at 0:19
Yeah, I may to still have to tweak it a little bit. I want to make it cycle (or rotate) hourly, or daily, between both while they're connected. Then I may update this answer. But thanks for following up! :) – cregox Feb 19 '10 at 14:08

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