Recently, several users have contacted me about inconsistencies between amount of free space that is displayed in Finder compared to other methods.
June contacted me with the following question:
“Since upgrading to Mountain Lion, my commercial disc utility reports that my MBP has 43GB less free space than is displayed in Finder. Could this be an error in my SSD drive?”
Luckily, this is unlikely to be an error with your hard drive or related to the fact that you are using a solid state drive. If you are ever concerned about drive or file system errors, running Disk Utility and Verify Disk on your drives is a good first step. However, the solution in your case is likely less concerning.
OS X keeps Time Machine local snapshots on your disk drive. In most circumstances these backups are ignored when the file system displays the amount of free space to the user. The disk space is actually being used, but for all purposes it is free for the user. This is because these backups are removed automatically as that disk space is needed by the user.
One way to see if this is your actual circumstance is to view the Storage tab under Storage section in the About This Mac application. Because I am pushing the limits of my hard drive space already, OS X has already removed local backups from my system. Users with “missing” hard drive space will often find it here as backups.
Remember, that OS X automatically handles this space and removes these files as needed. Therefore, this inconsistency can be ignored.
Most users are already aware of two other common reasons for the incorrect amount of space being reported. Files in the Trash may confuse disk size reporting. This can be tested by executing the Empty Trash command and rechecking how much space is reported. Additionally, since Snow Leopard, Mac now reports as base-10 (GB) versus the base-2 (GiB). This corrected the issue that a 400GB drive would seem to hold 360GB (actually GiB) in data. However, tools that continue to display as base-2 will show less space than the OS. This effect is magnified for larger drives.