I have a 3TB external drive, that for some reason is partitioned as a single Master Boot Record partition. Seagate, go figure.

I'm running a 1TB Mac with 10.9.4 and about 380GB used space. My backup drive that I need to repartition is using about 1.13 TB of the drive.

What I'd like to do is create a compressed image that I can save on my Mac, then repartition my external drive with the proper partitions, then use the image to restore everything back to the external drive.

So my questions:

  1. Will my 1.13 TB drive compress to a small enough size to fit on my remaining 628G Mac drive?
  2. What options (specifically) do I choose when creating the image so that it's easily restorable to the reformatted drive?

From Disk Utility, select the partition on the left (not the drive) you want to back up. Then on the menu bar, select File -> New Image From Disk. In the options, select the location where you want to save it and choose Compressed from the type dropdown. That will create a compressed .DMG file of the entire drive.

Will it fit in 628GB of space? That's an impossible question to answer. It depends on how compressible the data is. But there's a good chance that it won't. You'd need a 2 to 1 compression ratio to make it fit. If the contents of the drive are virtual disk files or database files, it probably would. But if it's mostly music, videos, and pictures, there's no way it'd fit.

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  • Thanks Wes. 750G of this drive is video. I store my iTunes media on this drive. Looks like I'll have to prune some older stuff, then move some to my laptop in order to create a file I can actually save. Thanks again. – Vince Kronlein Sep 18 '14 at 0:56

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