TimeMachine on multiple (smaller) disks ?

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bruxxx Author

Level 1

45 points

TimeMachine on multiple (smaller) disks ?

I just bought this ginormous 8TB Western Digital My Book RAID drive.

I loaded and restored my system to this drive and am booting to it, works great.

Now, I want to set up TimeMachine to back it up, and I have 2 4TB WD My Book Pro drives to use.

I can add both the 4TB drives to TimeMachine, but when I start the backup TimeMachine bails and

says it cannot backup because there is too much data on the 8TB to back up to a single 4TB drive

even though it has two 4TB drives to backup too.

This is confusing since my reading of this on the net says that it will place backup data wherever

it can using multiple disks ... so as long as the sum of the space on the TimeMachine disks is

greater than the backup dataset one would think this would work.

But it does not seem to.

Is there a trick to this, or somewhere in the docs that says I cannot really do this, that any

single backup disk must be bigger than the data I want to backup? What is the truth here?

Thanks.

iMac, OS X Mountain Lion (10.8.2), 24 inch

Posted on Sep 30, 2013 5:55 AM

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Answer 1

Best reply

Linc Davis

Level 10

209,541 points

Sep 30, 2013 8:02 PM in response to bruxxx

as long as the sum of the space on the TimeMachine disks is greater than the backup dataset one would think this would work

No. Each backup volume is independent of the others. The only way you can do what you want is to combine the two backup drives in a striped software RAID, which is undesirable in terms of reliability.

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Answer 2

bruxxx Author

Level 1

45 points

Sep 30, 2013 8:33 PM in response to Linc Davis

Linc ... thank you for such a fast and qualified answer. I've asked quite a few questions here and yours was the fastest definitive answer I've gotten, by far. A lot of the talk about this on the Internet implies that TimeMachine will write the same backup set to two disks. It may not be the best solution, but when you get up to large disk sets there is not much choice that I can see. I have a NetGear Pioneer Pro NAS box that has 12 TB on it ... but I would prefer not to have to use network bandwidth to back it up or dedicate a backup network at home.

I was tending to think that creating a RAID backup volume might be OK, since the probabilties are mostly on my side ... I mean the chances of the system disk and the backup disk going back around the same time are pretty small. However, these Western Digital MyBook boxes are internally RAID'ed as well, so the probability of failure however small is quadrupled, so we are in agreement on that.

What I was wondering is that if I cannot get everything in the same backup, can I create two backup jobs that in some way divide my system into 2 and backup half to one part of the disk set and the other have to the other part of the disk set? Thanks.

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Answer 3

Linc Davis

Level 10

209,541 points

Sep 30, 2013 10:51 PM in response to bruxxx

What I was wondering is that if I cannot get everything in the same backup, can I create two backup jobs that in some way divide my system into 2 and backup half to one part of the disk set and the other have to the other part of the disk set?

Not possible. Time Machine is designed for simplicity, not flexibility.

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