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I created a Read/Write DMG file from a folder in my Mac, size is about 175GB. Then I copied the DMG to an external HD for backup. To ensure data integrity, I checked the MD5 of both, local HD and external HD copies of the DMG file and they are different! I reformatted the external HD and copied the DMG onto it again and it was different again. I did notice that the first MD5 of the DMG in the external HD matched the MD5 of the second attempt in the external HD, so it looks like the file is being copied consistently.

After all this, I created a simple txt file on the local HD, copied it to the external HD and checked the MD5s, they match as expected.

Anybody know why the MD5 of the DMG on the local HD is not the same when copied to an external HD?

NOTE: both local and external are HFS+

Edit 2: I checked the sha1 on the said DMG files and they match... I wonder why MD5 does not match though. Still an open question.

  • both local and external are HFS+ – waffle Dec 15 '17 at 11:29
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    Since the sha1 hashes match, the contents pretty much have to be identical, so something's going wrong with the md5 calculation. Exactly how are you calculating the md5 hash? – Gordon Davisson Dec 16 '17 at 9:32

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