kmosx: Questions about Disk Utility

Q: "In Disk Utility's main window, I see two disks, which one am I supposed to select?"

A: The top icon represents your hard drive, while the nested icon represents a volume on that drive. See this image for more information. When you get your Mac from Apple, it'll have a disk with one volume, so it's not as evident for you as it is for me, where I have many different partitions (aka "volumes"). When you select the drive, the options that are shown to the right apply to the drive as a whole, which includes looping through each volume. (In the image, you can see that both the Verify/Repair Disk Permissions, and the Verify/Repair Disk buttons are disabled. This is because one of the volumes on that drive doesn't have OS X on it (which disables the ability to Verify/Repair Permissions), and another volume is the one I started up from (which disables the ability to Verify/Repair the Disk)). If you only have one volume, it doesn't make any difference which one you select.



Q: "What's the difference between Verify and Repair?"

A: Verify verifies. Repair verifies and repairs, if necessary. Usually not much sense in verifying, unless you only want to check to see if there are any problems without correcting the problems at the same time.



Q: "How does Disk Utility work when it repairs my permissions?"

A: The way Disk Utility works is that when you launch it and select a volume in the left hand column, Disk Utility looks to see what Verify or Repair options are available for that volume. If it's the boot volume, the option to Verify or Repair the Volume will be disabled. If the volume isn't a volume that has OS X installed, the option to Verify or Repair Permissions will be disabled.

When you choose a volume in the left hand column, and click the Verify or Repair Permissions button, Disk Utility looks to the /Library/Receipts/ folder for the volume which you have seleced and uses that during the verification or repair. This is the way Repair Permissions has always worked, ever since OS X 10.2.0. Unfortunately, most of the Mac community has been led to believe otherwise, in that they believe that the /Library/Receipts/ folder of the startup volume was always used during the repair, even if you selected a volume in the lefthand column that was not the startup volume. Unfortunately this is incorrect. Disk Utility always uses the /Library/Receipts/ folder for whichever volume you've selected in the lefthand column.

In this /Library/Receipts/ folder, are a series of package receipt files, with a .pkg filename extension. What these are are a receipt for any item you've installed using Apple's Installer application, or installed through the Software Update preference pane. They contain a list of the files that were installed, and the "proper" permission settings of those files. When you choose to verify or repair permissions, Disk Utility (actually, the secondary 'Disk Utility Agent' process in Jaguar, or the 'DiskManagementTool' in Panther) looks to this folder and the receipt files, and forms a conglomerate list of all the permission settings defined therein. This forms the base set of permissions that Disk Utility uses during verification or repair.

In the past, Apple has updated the Disk Utility application, or the frameworks it depends on, by "hard-coding" the permissions for certain files, thereby overriding the base set of permissions mentioned above. These new permission settings are referred to within the infamous "We are using special permissions for..." line when running the Disk Utility's Verify or Repair Permissions function.

In the case of OS X 10.3.3 or greater, the following file contains a "specificFileSettings" entry for "./System/Library/Filesystems/cd9660.fs/cd9660.util" with a value of "33261" (click the link to locate the file in the Finder):



That message will be there from now on, every time we run Repair Permissions. (Many users seemed to be confused or alarmed about who this "we" refers to. Well, "We" refers to the aliens from outer space which have taken over your computer. :-D Actually, my guess would simply be that "We" could refer to Apple, since they've created the software you're running, or it could refer to Apple and you, since you're working together to verify or repair your permissions. All in all, the wording isn't worth losing sleep over . So, Disk Utility then combines this special setting with the base set of permission settings, and uses it to verify or repair you hard drive.

Repair Permissions has never been, and never will be, an exact science. After all, OS X went two full versions without any such feature built-in to Disk Utility. That should hopefully tell you something. Instead, Apple designed Repair Permissions to simply be a convenient way to make sure that the permission settings of the some 100,000 files on your hard drive don't get too out of whack. Repairing Permissions is not as black-and-white of an issue as many users have been led to believe, in that the permission settings for your files aren't either right or wrong; rather, there can sometimes be a gray area. The fact is, your Mac would still work as designed if the permission settings of many files were slightly different than what Repair Permissions say they should be.



Q: "Should I Repair Permissions from my hard drive or from my Install CD/DVD?"

A: Well, as we've learned, Repair Permissions is driven off the /Library/Receipts/ folder of whichever volume you've selected in the lefthand column. Later updates to OS X have changed the preferred setting of a single file (See the link referenced above). Therefore, it's not a big deal if you were to repair the permissions off the CD, or even the permissions of another volume with a different version of OS X on it (such as the one partition with 10.2.6, another with 10.2.8, and another with 10.2.8 and the latest security updates that I have on my machine). The only differences found would be the single "hinted" file, and, of course, the same problems Repair Permissions seems to find every time. That said, you'd probably agree that it doesn't make a whole lot of sense to repair permissions from the CD, since the setting on that one file will be different from what Apple would like it to be. Obviously, it's not going to break the OS, but it basically makes more sense to repair permissions off the hard drive.

Situations can arise in which is isn't possible to repair permissions successfully from the hard drive. In Jaguar, the permission settings of the Disk Utility application itself could become corrupt, so that it'd be unable to successfully repair your drive (or even itself) if it needed to. I wrote a freeware application, Repair Disk Utility's Permissions that could fix this problem in Jaguar. That same type of problem can also be solved by running Repair Permissions from the install CD.

Hope this helps....