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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, superuser.com/questions/342259 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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