How to fix Disk Utility, About This Mac, and Finder that disagree on hard drive space usage?
Most of the difference you're seeing is due to Time Machine's "Local Snapshots" feature. When Time Machine is turned on but the backup device isn't available, it backs up to the local volume. The space used for these local snapshots is counted as "in use" by Disk Utility and System Information, but not the Finder (see the Disk Space considerations section of this article). While the space for backup actually is in use, it'll be freed automatically when needed (i.e. when the volume gets above 80% full), so the Finder counts it as being available.
In your particular case, the space System Information lists as being used for "Backups", 98.36GB, is very close to the difference in free space listed by the Finder vs. the other two. The Finder's 206.43GB free - 98.36GB of backups = 108.07GB actually free; compare to System Info's listing of 108.44GB free and Disk Util's 108.31GB. I'm not sure what the rest of the difference is (maybe they looked at the disk at slightly different times? Or they may be counting volume structures a bit differently?), but it's very small.
Finder shows:
- easily, the truth for JHFS+ volume Macintosh HD
- optionally, the truth for mtmfs volume MobileBackups
Hint: with local snapshots enabled in Time Machine, in Finder you can go to /Volumes/MobileBackups then get info.
Results of a one-line command show that for some purposes, MobileBackups is treated as a distant file system:
qlmanage -m disks | grep MobileBackups && mount | grep MobileBackups
As Disk Utility is oriented to local file systems, we'll probably never see MobileBackups as a separate volume in that context. There are degrees of simplification in Disk Utility, even when debug options are chosen.
It looks like Lion changed a few things about disk space management, and you're not the only person with this issue and likely related issues. Some are reporting that doing a shutdown and restart resolves the discrepancy. Others indicate their disk space discrepancy had to do with Time Machine's local backup feature on laptops.
Do a full reboot and see if that resolves the problem. If not, disable local backups, then do a full reboot.