There are several ways to display disk usage in OS X, with some of the more-common options being the Finder's status bar, or to get information on your hard disk in the Finder, or to use Apple's Disk Utility program. However, while these options will display the available disk space on the system, in OS X Lion you might see some differences between the numbers reported in these areas.
For example, on my Mac running OS X Lion there is currently 184.43GB of free space reported as being available on the disk if I get information on my hard disk in the Finder. This same number is reported below the Finder window in the status area, which can be enabled in the View menu or by pressing Command-slash.
Even though these two areas report similar available disk space, if I go to Disk Utility and select "Macintosh HD" in the device list, it is apparent that this number is not the same in this program. On my system, instead of the expected 184.43GB as the available disk space, Disk Utility reports about 2GB less, at 182.34GB. This same value is repeated elsewhere in the program, such as in the properties listed when you get information on the disk and scroll to the "Free Space" section.
This discrepancy might be confusing to people, but overall the differences in these values does not mean much for the end-user. In fact, in an odd way both values are correct.
The reason for this difference is Apple's move to include local backup snapshots on the system in Lion, which allow you to access recent backups of files without needing to have your Time Machine disk updated and attached to your system. When you delete a file, the system will still retain it in a virtual disk called "MobileBackups," which you can see if you open the Terminal and type the following command:
This command will list the contents of the hidden "Volumes" folder at the root of your hard drive, which will contain your boot drive and any additional drives that the system has mounted (both external and internal). In addition, this folder is used as the mount point for software-based disks such as opened disk images, network shares, and, in Lion, Apple's virtual drive for holding the mobile Time Machine snapshots.
If you open the MobileBackups virtual disk in the Finder, you will see a similar data structure on it as you would see on the Time Machine drive, with a main "Backups.backupdb" directory followed by a folder for your computer, which contains individual backup instances.
The contents of this MobileBackups directory is the reason for the discrepancy between the Finder and other tools like Disk Utility -- the Finder omits it from its Available Space calculation, and Disk Utility does not. Because the MobileBackups volume is stored on your hard disk, it is currently taking up space on your drive so technically Disk Utility is correct in reporting a lower amount of available space; however, the Finder is also correct in its calculation of a higher value.
The reason for this is the contents of the MobileBackups disk will be dynamically adjusted to accommodate the space needs on the drive, so the files in it are almost like a temporary cache rather than a set of files that will need to be removed by the user in order to relinquish more space. Therefore, as you use the system and fill up your drive, Time Machine will empty the backups it stores and allow that space to be used, if necessary.
If you go to the Apple menu in Lion and choose "About this Mac," followed by clicking the "More Info" button, you will open the "System Information" utility that will show you a default user-friendly window that contains statistics and other information about your system. If you click on the "Storage" section you will see the breakdown of disk space usage, with one of the categories being "Backups." If you include the size value of the Backups in your comparison between the Finder's Available Space calculation and that in Disk Utility, then you will see it matches the difference. In my case, the difference between 184.43GB and 182.34GB is 2.09GB, which is the exact amount reported as being used for backups on this system.