Disk Images in OSX

Apple’s Disk Utility

Apple’s Disk Utility is a very versatile tool included with OSX for working with hard disk drives and optical drives as well as disk images. Though the exact features varies with the OS release, it is a rich utility that could be covered much more extensively than is the purpose here. For example, Disk Utility allows for:

  • Secure erasing, formatting, and partitioning of disks and optical media (e.g., DVD-RW)
  • Repair file systems and permissions
  • RAID support for mirrored, striped or concatenation
  • Manual ejection of removable media (this comes in handy)
  • Create/verify MD5 and CRC32 hash/checksums

Disk Utility, in accordance with its image management features, maintains a list of images with which it has worked for easy access.

To launch Disk Utility, launch it from Finder’s Applications -> Utilities -> Disk Utility.

Image Creation and Manipulation

To create a DMG from a disk

  1. Select the media in the top left pane of the Disk Utility (though you can select “slices” as well).
  2. Select File -> New -> Disk Image From… OR click the New Image button from the toolbar (a document with a disk and a plus sign on it).
  3. Enter in an image name (such as the name of the disk)
  4. By default, DMGs are not encrypted. If you require it, choose the encryption strength in the encryption selection box.
  5. Click Save to begin the image creation process.
  6. Now – now,  not later -is the time to CREATE an MD5! Don’t wait!
  7. Trust me on the prior point – corrupt images are bad. I will always, always, always emphasize this. :-)

To create an ISO from a DMG:
hdiutil convert diskimage.dmg -format UDTO -o diskimage.iso

To create an ISO from within Disk Utility:

  1. Select the media in the top left pane of the Disk Utility (though you can select “slices” as well).
  2. Select File -> New -> Disk Image From… OR click the New Image button from the toolbar (a document with a disk and a plus sign on it).
  3. Enter in an image name (such as the name of the disk)
  4. Choose “CD/DVD master” from the “image format” selection box. Do not use encryption.
  5. Click Save to begin the image creation process.
  6. After the .cdr file is created, in Finder (or shell) change the extension to .iso. Finder will warn you and give you a chance to retain CDR or to really continue the change to ISO: Just select ISO.
  7. Now – now,  not later -is the time to CREATE an MD5! Don’t wait!
  8. Trust me on the prior point – corrupt images are bad. :-)
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