Disk image copied every time
 

I have some huge compressed DMG that I mount often, but not even their "last opened" date changes. Yet SD copies them every time anew. What piece of metadata am I overlooking?

I don't know. I assume they don't copy if you don't open them?

Yes. But they don't always copy even if I open them. Whether using DiskImageMounter or hdiutil makes no difference. The file format is read-only and the files are locked, so it is a mystery why they are considered changed.

I can only suggest that they are, indeed, changing (or their metadata is).

After verifying a disk image the inode of the file and UUID of the volume are written to an extended attribute. So merely opening a DMG on the target makes it different in the eyes of SuperDuper.

This is unfortunate as I like the checksum of compressed images to alert me to hardware failure. A workaround is locking the DMG to prevent the attribute from changing, but then the image is verified each time you open it.

Does SD already special-case some metadata? If so, how about ignoring com.apple.diskimages.recentcksum? If it doesn't make things too slow.

The above doesn't explain why my locked images are still copied, but I'm fed up with experiments that take hours. Is there a dry-run mode in the command line version of SD?

If the metadata changes, then the file will be recopied - we don't just update metadata at this point.