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Lesson 4. File Systems > Working with Disk Utility

Working with Disk Utility

Disk Utility is a very useful tool for working with volumes. Beginning with Mac OS X 10.3, Disk Utility includes the functionality provided by Disk Copy in previous versions of Mac OS X. Disk Utility's user interface has changed to accommodate the feature set of both programs. This section discusses the various features of Disk Utility.

Disk Utility is the primary tool you will use for volume and disk management. You will find Disk Utility on the Mac OS X Install DVD and at /Applications/Utilities. You can run Disk Utility before an installation to perform a number of tasks: to check, repair, partition, and format drives; to create images of existing data; or to mount or unmount and restore existing partitions. Disk Utility can also be used as a diagnostic tool. Note that you cannot erase or partition the startup disk.

You will immediately notice disks and partitions in the list at the left when you open Disk Utility. If you select a disk, you can use the Partition pane to create partitions. You cannot partition a partition; so the Partition pane is only accessible if you select a disk. In the Erase pane, you can erase the contents of a partition or drive and change its format. You can also use this utility to erase CD-RW and DVD-RW discs.

If you plan to boot into Mac OS 9, and your computer supports it, you must select the Install Mac OS 9 Disk Drivers checkbox when you partition your hard disk. This formatting option places Mac OS 9 startup drivers on the disk. If these drivers are not present, you will not be able to start up the computer in Mac OS 9. This option is not required for using the Classic Environment in Mac OS X, because Classic can access drives in the standard Mac OS Extended formats.


The Mac OS 9 driver option is hardware dependent and does not appear on systems that do not start up in Mac OS 9, such as the Mac mini.

Getting Information

There are two easy ways to get in-depth information about the drives and volumes connected to your computer: System Profiler and Disk Utility. Each provides very complete information about your drives and volumes, although the information is presented in different contexts.

System Profiler (/Applications/Utilities) is bus-oriented, and when it provides information about storage devices, it identifies them in the context of the bus on which they are located. When you select the drive bus on the left, you see all devices on that bus (including hard drives, optical drives, and other connected devices).

System Profiler is a reporting tool rather than a utility that edits your disks and partitions, so it provides no information about unmounted partitions on a particular disk.

Disk Utility is drive-oriented, so the left column in Disk Utility contains a list of all connected drives. Beneath each drive is an indented list of partitions on that drive. Starting with Mac OS X 10.3, CDs recorded with multiple sessions are displayed correctly as separate partitions in the left column of the Disk Utility window. Unmounted partitions are dimmed, and information about them cannot be gathered.

When a drive or partition is selected from the list in Disk Utility, you can click Info to display detailed information about the drive, such as the drive's capacity and bus type.

Applying First Aid

The First Aid pane of Disk Utility can repair permissions issues and disk format problems. Unusual behavior, such as inability to mount disk images, copy files, install applications, or spool print jobs, could be caused by incorrect file permissions. Missing files and folders could be caused by disk format problems.

The Repair Disk Permissions button restores the default permissions of Mac OS X system files and applications in the Applications folder on the startup disk only. Disk Utility restores the default permissions of items installed by the Apple installer based on information in the installation receipt packages stored in /Library/Receipts. However, many third-party installers do not store installation receipts. This means many applications installed with third-party installers, and any applications installed by drag-and-drop, will not be affected by the Repair Disk Permissions feature. While Disk Utility does not fix permissions on those files, it is easy to manually correct file permissions, as you will see in later lessons.

The Repair Disk feature verifies and repairs the folder structure of a volume, using the BSD tool fsck. Disk Utility can perform repairs on partitions in all of the formats that it can create: UFS, Mac OS Standard, Mac OS Extended, and MS-DOS (FAT).

Because you cannot repair your startup disk while your computer is booted from it, you would normally use Repair Disk from your Mac OS X Install DVD. You could also repair your startup disk using a second computer, with your computer connected in Target Disk Mode.

If Disk Utility indicates that it discovered problems after you click Repair Disk Permissions or Repair Disk, repeat the process until you get a clean bill of health.

Remember, the options available in the First Aid pane are just that—your first line of defense against minor disk injuries. For more serious trauma cases, you'll want to rely on more powerful third-party tools.

Using Secure Erase

The Erase pane in Disk Utility allows you to securely erase free space or an entire volume. If you wish to ensure that files previously placed in the Finder's Trash are completely erased, start by clicking the Erase Free Space button. If you wish to erase an entire drive or partition, start by selecting it and clicking the Security Options button.

Whether you are securely erasing free space or an entire volume, Disk Utility's options are fairly straightforward. In both methods, you can select Zero Out Data, 7-Pass Erase, or 35-Pass Erase. Zero Out Data writes a single pass of zeros over the portions of the disk being erased, while 7or 35-Pass will write and delete random zeros and ones for the specified number of passes.

While the multipass erase options take time to run, they provide a highly secure mechanism for ensuring that the data on a drive cannot be restored. This is very useful when retasking a computer or transferring a drive between employees with different levels of security access, or clearing a home machine for a change of ownership. Allocate plenty of time for a multipass erase.


7-Pass Erase meets current U.S. Department of Defense security requirements. Use 35-Pass Erase for very sensitive data.


The Finder also supports a Secure Empty Trash option that performs a 7-Pass Erase on items in the Trash. Files erased this way do not need to be erased again with Erase Free Space unless you wish to apply more erase passes to the files.

Configuring RAID

Disk Utility provides a tool to configure a software RAID in Mac OS X. Redundant Array of Independent Disks (RAID) is used to configure multiple hard disks so that they appear as one volume in the operating system (sort of the opposite of partitioning, which makes one hard disk look like multiple volumes). You can configure the RAID scheme to use concatenation or striping (RAID 0), which stores data across the disks, or mirroring (RAID 1), which stores the same data on all disks. With RAID 0 selected, a single volume will display the drive capacity as the total amount of all drives being used in the array.

This illustration explains how an 11 GB file is saved to disk under the different RAID schemes. At the left, two 10 GB drives act as one regular-speed 20 GB volume with concatenation. Center, two 10 GB drives act as one fast 20 GB volume with striping. Right, two 20 GB drives act as one 20 GB mirrored volume. As always, all disks in a RAID should be the same size, as RAID will use the lowest-common denominator size (30 GB and 20 GB striped together would make a 40 GB volume).


New in Mac OS X 10.4 is the ability to use partitions in creating a RAID.

  • Concatenation (RAID 0)— The most basic form of RAID 0. This form allows multiple drives to be treated as a single volume, although without performance benefits or redundancy. In the Finder, two concatenated 10 GB drives appear as a single 20 GB volume. Data written to this volume is written to the first drive until it is full, and then written to subsequent drives in the RAID as needed. Concatenated RAID volumes are often referred to as “JBOD” (Just a Bunch of Disks).

  • Striping (RAID 0)— Allows multiple drives to be treated as a single volume, although data is striped across all disks. This allows performance improvements because each drive can access the bus separately, improving throughput.

  • Mirroring (RAID 1)— Stores the same data on all disks, allowing redundant data storage. However, although RAID 0 drives are added together to create the full volume size available in the Finder, a mirrored RAID will appear in the Finder to be the size of the smallest physical disk.


Each RAID format has risks and advantages. Keep in mind that mirroring is the only RAID format that provides data redundancy.

Exploring Disk Utility

In this exercise, you will explore the features available in Disk Utility.

Log in as Chris Johnson.

In the Finder, choose Go > Utilities (Command-Shift-U).

Open Disk Utility.

Select the entry in the list that represents your main hard disk.

The disk entry usually has the manufacturer name visible. Do not select the formatted drive partition, the entry that is indented beneath the drive entry. Note that the Partition button is available when you select a disk.

The bottom of the screen displays details about the disk drive, such as its total capacity and its connection bus.

Note the capacity of your main disk drive.

Note the connection bus used for your computer.

Your connection bus would depend upon the computer you are using, but would most likely be either ATA or Serial ATA. Less likely buses would be SCSI, FireWire, or USB.

Select the partition entry beneath the disk drive entry.

Now that you have selected a partition, the Partition button is no longer available, because you cannot partition a partition. Also notice that the information at the bottom of the screen has changed to display details for the selected partition. Disk Utility displays the number of files on the partition, the partition's format, and where in the file system the partition is mounted. If you have selected the boot partition, the mount point is /.If you had selected a non-boot partition, the mount point would be /Volumes/name, where “name” is the name of the partition.

Click the First Aid button.

The First Aid pane allows you to verify and repair disk permissions and the disk itself. Because you have selected the partition from which you are booted, the Verify Disk and Repair Disk buttons are dimmed. In order to repair your main hard drive, you would need to boot using another disk, such as the Mac OS X Install DVD.

On the other hand, the Verify Disk Permissions and Repair Disk Permissions buttons are only available when you have selected a boot volume.

Click the Erase button.

The Erase pane allows you to erase free space on a partition or completely erase a disk or partition. In this case, you have selected your boot volume, so the Security Options and Erase buttons are dimmed. You could erase free space.

Click the RAID button.

The RAID pane is used to combine two or more volumes into a RAID volume.

Click the Restore button.

The Restore pane is used when you want to restore applications and files from a disk image.

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