Disk-Arbitrator | by Aaron Burghardt


by Aaron Burghardt


Disk Arbitrator is a Mac OS X forensic utility designed to help the user ensure correct forensic procedures are followed during imaging of a disk device. Disk Arbitrator is essentially a user interface to the Disk Arbitration framework, which enables a program to participate in the management of block storage devices, including the automatic mounting of file systems. When enabled, Disk Arbitrator will block the mounting of file systems to avoid mounting as read-write and violating the integrity of the evidence.

It is important to note that Disk Arbitrator is not a software write blocker---it does not change the state of currently attached devices nor does it affect newly attached devices to force a device to be read-only. The user still must be careful to not accidentally write to a disk with a command such as "dd". Owing to this fact, a hardware or software write-blocker may still be desirable for the most sound procedure. Disk Arbitrator compliments a write-blocker with additional useful features and eliminates the typical forensic recommendation to "disable disk arbitration."

System Requirements

  • Intel Mac
  • OS X 10.5 or later


You can find links to compiled executables on the releases page.

Quick Start


To install, drag the Disk Arbitrator application to the desired location, for example /Applications.

You may optionally want to have Disk Arbitrator automatically running every time you log in. There are two ways to do this:

  • Add Disk Arbitrator to your Login Items in the User & Groups (or Accounts on older OS X versions) preference panel in System Preferences.

  • Use the included "Install User Launch Agent" feature (accessible from the menu). When installed, the system's launchd will automatically launch Disk Arbitrator when you log in, just like a Login Item. In addition, the plist contains a setting which instructs launchd to monitor the application and automatically relaunch it in the event of a crash or if otherwise quit. This offers the most assurance that Disk Arbitrator will be running whenever you are logged in.


When launched, it adds its icon to the status bar on the right side of the menu bar. The status bar icon indicates one of three states:

  • Green: the utility is activated and in Block Mounts mode.

  • Orange: the utility is activated and in Read-only mode.

  • Gray: the utility is deactivated and attached disks will be automatically mounted by the system.

Disk Arbitrator continuously monitors for disks to appear and disappear and tracks the disks in the main window. When a new disk is attached, the system notifies Disk Arbitrator and gives it a chance to reject mounting of a disk volume. Disk Arbitrator responds as such:

  • When deactivated, it just observes the disk changes

  • When activated and in Block Mounts mode, it simply rejects every new system attempt to mount a volume.

  • When activated and in Read-only mode, it rejects the original mount action and automatically sends its own request to mount the volume, but it ensures the mount includes the option to make the file system read-only. It also checks the file system type and, if it is HFS, it includes the flag to ignore the journal.

Reminder: Disk Arbitrator does its work by actively participating in the mounting process. If the utility is deactivated or is quit and not running, there is no protection from auto-mounting attached disks. However, once a disk appears and the process of either mounting or rejecting the mount is finished, then Disk Arbitrator may be quit without affecting the state of the disk.

Working With Disk Images

As of version 0.3.0, Disk Arbitrator has support for disk images.

Using Disk Arbitrator's Attach Disk Image feature, attaching the disk image is effectively coordinated. When "Attach Disk Image..." is selected from the menu, an open panel appears which includes additional options for attaching and mounting the disk image. The default is to only attach the disk image. When the "Open" button is clicked, Disk Arbitrator attaches the disk using hdiutil. Once it is attached, Disk Arbitrator's normal behavior applies, so if it is activated and the mode is set to Read-Only, the volumes on the disk image will be mounted read-only. If the mode is set to Block Mounts, the volumes will be ignored.

The disk image open panel includes an option to attempt to mount the disk image for when Disk Arbitrator isn't activated and is being used as a convenient means to attach a disk image. When the mount option is used, Disk Arbitrator passes "-mount optional" to hdiutil, which attempts to attach and mount the disk image, but with the benefit that if the mount fails, the disk image is not detached.

Disk Arbitrator also supports drag and drop to attach a disk image. Simply drag one or more disk images from a Finder window to Disk Arbitrator's list of disks to initiate attaching the disk images. When using drag and drop, the default options include "-mount optional", so mounting is also attempted. If the mount fails, the disk image remains attached and can be mounted manually.


  • Attempting to attach and mount a disk image outside of Disk Arbitrator, either by double-clicking it in the Finder or using hdiutil attach, will behave as before: the disk image will be attached, then the system will attempt to mount it, Disk Arbitrator will reject the mount, and the disk image will be unattached because the mount failed.

  • If a disk image with a Software License Agreement is attached, Disk Arbitrator automatically replies "Yes" to the agreement. Caveat Emptor.

A Note On Dirty Journals

When set to Read-only mode, the mount request that Disk Arbitrator sends includes the option to ignore the journal, which is useful when the HFS file system was last detached without unmounting (e.g., the system crashed, or an external drive was unplugged without ejecting it). If you are working with a disk that was not cleanly ejected, then attempts to attach it read-only will normally fail because HFS knows the journal needs to be replayed and it is not allowed to make the changes, so it fails to mount.

Mounting a disk image with a dirty file system can be achieved by using a shadow file, but this is less than ideal. The shadow file protects the original disk image from changes, but the file system is mounted read-write. A better option is to execute the two steps manually, using Terminal:

  1. hdiutil attach -nomount disk_image.dmg
  2. mount_hfs -j -o rdonly /dev/diskx /mount/path

Disk Arbitrator provides a convenient way to execute the second step, and a future version will perform both steps in one operation.

About the Author

Aaron Burghardt

The article was originally published at: https://github.com/aburgh/Disk-Arbitrator

August 29, 2022
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