While trying to clear space from my MacBook Pro hard drive, I noticed a 12.4 GB discrepancy between the amount of free space shown in Storage Bar in About This Mac and the total sum inside my Macintosh HD using OmniDiskSweeper (or du -sh * in Terminal). I’ve managed to account for 9.1 GB of that by running ODS as the root user.

Where could the remaining 3.3 GB be?

Can any of the major offenders be deleted to free up space? They are:

  • /System/Library/Caches/com.apple.coresymbolicationd/data (4.3 vs. 0 GB)
  • /.DocumentRevisions/.cs/ChunkStorage/0/0/ (1.5 vs. 0 GB)
  • /private/var/folders/zz/ (1.3 GB vs. 9.5 MB, mostly in the file CFNetworkDownload_L4uNTH.tmp)
  • /.MobileBackups/Computer/ (1.1 vs. 0 GB)
  • /.Spotlight-V100/Store-V2/ (0.6 vs 0 GB)
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    The reason that some of this isn't reported, is because macOS will remove most of this if space is needed, making the reporting of its existence pointless for most people. – William T Froggard Jul 7 '17 at 20:59
  • That doesn't seem to be true, as far as I can tell. I asked this question because I kept getting the message that my startup disk was full, and as soon as I looked into the problem I noticed the discrepancy. – David Russell Jul 9 '17 at 2:08
  • I didn't say it works as it should. I've noticed that macOS has issues actually carrying this out, but the code is in there to do it. Perhaps Apple will actually make it work correctly at some point. – William T Froggard Jul 9 '17 at 3:45

You can certainly delete the two temporary items in your list:

  • /System/Library/Caches/com.apple.coresymbolicationd/data/
  • /private/var/folders/zz/CFNetworkDownload_L4uNTH.tmp

Avoid deleting the folder structures but large files in these locations can be safely removed.

To be extra safe, restart your Mac first and then remove any files. The restart will ensure the files are not in use.

  • From what I've read, the .tmp is deleted upon rebooting, and indeed that space seemed to clear up when I rebooted. As for the com.apple.coresymbolicationd stuff, this seems to be useful for debugging and analyzing crash reports. So does deleting it render my crash reports useless? (My computer does crash.) – David Russell Jul 9 '17 at 2:13
  • You can safely remove the contents of Caches/com.apple.coresymbolicationd. Any information stored within a Caches folder is deemed temporary and non-critical. The contents of a cache will be recreated if needed. – Graham Miln Jul 9 '17 at 7:32

Getting rid of these files is a great move, but keep in mind many of these caches will regenerate. For instance, spotlight files will regenerate at a cost of high processor usage and noisy fans (in a MacBook) immediately (look out for mds in Activity Monitor). MobileBackups is backup data from your iPhone which you might want on hand if you lose your phone and which will probably replicate the next time you plug it in.

Don't forget that if you sleep your computer, you'll often end up with a sleep image the same size as your physical drive. As we are almost all using flash drives now, don't forget flash drives need about 20% free disk space for self-cleaning and maintenance (I'm thinking of the SSD's which do their own garbage collection like Samsung and most of the Intel SSD). Spinning disks can go down to 10% but anything saved after about 50% usage is 1/3 or more slower than peak performance.

Clean out the files but think about upgrading the disk or moving some files or applications off to external storage. Time to go and follow my own useful advice. We humans generally procrastinate on important but not urgent tasks.

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    The sleep image has the size of all inserted RAM modules (or less): 16 GiB RAM → 17.18 GB (or less) sleep image – klanomath Jul 8 '17 at 15:47
  • From what I've read, it seems that MoblieBackups is actually backing up my computer, not my phone. I almost never connect my phone to my computer. I don't understand what a sleep image is and how it relates to my question. Your point is just that I need to keep 20% space free? – David Russell Jul 9 '17 at 3:02
  • My point was that even if you find these mystery files and delete them, there's a good chance that the OS will regenerate them. As these mystery files may be keeping tabs on us (Windows XP and older used to keep a plain text log of all our text and emails in a hidden partition), I'm keen to know what files are using that space as well. – Foliovision Aug 3 '17 at 22:48

The /.MobileBackups folder is created by Time Machine, which saves snapshots to your local disk when your Time Machine volume is not available. To delete those snapshots and free up the space, the recommended way is to turn off local Time Machine backups altogether, by issuing the command:

sudo tmutil disablelocal

You can always turn them back on again later if your requirements change.

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