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My disk space usage doesn't add up. I should have used only about 80 gigabytes, but according to DaisyDisk about 25 gigabytes is taken up by hidden space. The mystery disk space seems to be classified as other in the About This Mac window.

I've disabled local Time Machine snapshots with sudo tmutil disablelocal and deleted /Volumes/MobileBackups/.

Does anyone know what the hidden space could be or how I could free it up?

Possibly related to Macbook HD "Other" all the Sudden Almost Full.

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How many users do you have? DaisyDisk might be unable to access a different user's directory. –  Vlad Mar 21 '12 at 9:13
    
@Vlad Just one. And the DaisyDisk (and du) results in the screenshot were scanned as root. –  ؘؘؘؘ Mar 21 '12 at 9:17
    
What does df -h output? –  Vlad Mar 21 '12 at 9:22
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Reboot into Recovery HD and run a disk repair in that environment. Sometimes there is corruption in the volume of the file system. That should fix it or at least identify any problems if they exist beyond just having content taking up space unnecessarily. –  cksum Mar 21 '12 at 9:31

4 Answers 4

up vote 5 down vote accepted

Starting up from the recovery partition and repairing the disk fixed the issue. (Disk Utility showed an error about an invalid free block count.) Thanks @cksum.

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From what I can tell reading the documentation and running the program on several different machines is that the hidden space captures storage inefficiencies by subtracting the expected space from each allocation that was measured and providing the difference as "hidden space". I wonder if your unallocated block counts were the cause or if it was more severe catalog and filesystem corruption to make 25 GB "go missing" –  bmike May 14 at 17:08

Another possibility—and one that ended up being the culprit in my particular case—is that your machine is making local backups when it's unable to connect to your external backup source. In this case, you can just disable the backups by entering the following command in the Terminal:

sudo tmutil disablelocal

Later, if and when you decide to re-enable the backup, you can reverse it with the following Terminal command:

sudo tmutil enablelocal

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DaisyDisk's hidden space (if scanned as administrator) should be within 4-8GB limit for most Macs. If the numbers are larger, try repairing the volume as this may serve an indicator for file system errors.

Taras Brizitsky, DaisyDisk's designer.

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Welcome to the site. Feel free to add your information to your profile and thanks for self-disclosing your relationship to the product as that is encouraged to provide transparency in case of recommendations, etc... –  bmike May 14 at 17:05

According to DaisyDisk manual:

Sometimes the amount of "used" space on a disk may appear greater than the total size of all scanned files and folders on that disk. This is normal. The difference is usually taken by the file system overhead and system stuff like Spotlight index, but in certain cases a significant amount of disk space may be hiding in other users' accounts or folders that are normally not visible to you. In order to reveal those folders you should scan the disk as administrator.

(Hidden space) is a virtual disk map item that only appears when the total size of restricted files is significant, so you can see whether it's worth rescanning the disk as administrator or not.

Large amounts of hidden disk space may also indicate to file system errors. Follow our troubleshooting guide for details.

In case you have a lot of hidden disk space when running it as administrator, it's possible that you have file system errors or broken file permissions.

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