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"The disk usage on the target doesn't match the source -- did CCC miss some files?"

 

There are a couple legitimate explanations for a mismatch between the capacities reported in Disk Utility. First, when cloning using the "Incremental backup of selected items" cloning method, there is a list of items that are excluded from the backup either because they are ephemeral items regenerated every time your machine reboots, or they are not appropriate to back up or because they actually reduce the portability of your OS to another computer. An exhaustive list of those items is included in the CCC Help section labeled "Configuring filters in CCC to limit what is backed up".

While I don't recommend it, you could modify the list of items that CCC ignores by editing the "defaults.plist" file within the CCC application bundle. Note that these items are not ignored when using the "Backup everything" cloning method (that cloning method references a smaller, very conservative subset of items, also referenced in the defaults.plist file).

While the exclusion of these items explains much of the disk capacity discrepancy you may discover in Disk Utility, it does not explain it all. A user suggested the following scenario after performing a "Backup everything" clone to his target drive:

    Boot drive - 39.67 GB used
    Backup drive - 38.55 GB used

Should I be concerned there's more than a GB missing? Could this be a swap file or something? Is there an easy way to isolate what files are not on both?

One GB seems like a lot, but it's not surprising, not for 40GB used (the discrepancy increases with the amount of data on your boot volume). Discrepancies like this are not uncommon when you're booted from the source or (subsequently) the target.

The issue is that Disk Utility (and Finder Get Info for the volume) is misleading. The value that Disk Utility reports is indeed the amount of space consumed on your hard drive, however, it is not the amount of space consumed on the drive by all the files and folders that you can see. And I'm not referring to the other files as simply "invisible", these other files and directories that make up the rest of the space you're "missing" are simply not presented to the operating system. These items are filesystem implementation details, and it isn't possible to copy them directly with file-level copying tools.

Does this mean you're losing data? Absolutely not. It's pretty easy to prove it to yourself too, just boot from your cloned volume and take a look at the capacity reports in Disk Utility. Here's an example we performed on a test machine:

** Booted from the original source volume
Source: 5,258,776,576 bytes
Clone: 5,025,562,624 bytes
**Booted from the clone volume
Source: 4,996,599,808 bytes
Clone: 5,250,097,152 bytes

Weird, eh? Having stared at this issue for a considerable amount of time, and having developed a utility that would carefully analyze every single file that I could find and copy, we're 100% confident that we aren't losing a scrap of information with CCC's file-level copy engine for the "Backup everything" cloning method. What we have learned and we're confident in saying is that Disk Utility can't really be used as a good measure of success for your clone. That's not to say that what it reports is wrong, it is simply misleading.