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So I have backed up my Macintosh HD at different instances to DMG files. I would like to compress them but when I try to make an archive it simply compresses each image, without recognizing the similarities in the content of the images (only a few GB of differences in ~70GB of data). The resulting size of the archive is about the same as the sum of sizes of a compressed DMG for each backup (I checked). I don't have them compressed or encrypted as I hoped this would help. I wonder if there is a way to get a compression program to recognize the duplicate content in the uncompressed disk images. Would creating a compressed DMG with 2 partitions (one for each backup of my original disk) work? If so how would I go about that? There must be a way to do this...

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If making a new DMG is not out of the question, might be of some help to you. Another way would be to compare the contents of your two DMGs, and delete the duplicate files from one before compacting/compressing it. (The "compact" only works if the DMG is a sparse image file). Simplest(?) of all would be to connect an external HDD and let TimeMachine manage all the backups for you. – Kent Jun 13 '13 at 5:44
Are you sure they're not compressed? Exactly which of the .dmg disk image formats are they, then? Use hdiuitl imageinfo filename.dmg to double-check. Are they UDRW? UDRO? – Spiff Jun 13 '13 at 7:38

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