How to Create a Disk Image in Mac OSX. A disk image is a file that contains the contents of a DVD, CD or even hard drive. It is a convenient way to back up the data found on disks or transport the contents of a disk electronically. OSX commonly uses .CDR and .DMG files. A .CDR file can be renamed with the .ISO extension and used by Windows equipped PCs.
Insert the disk to convert into the disk drive. Wait for the computer to identify the disk, and add an icon for it on the desktop.
Double click the Macintosh HD (or main hard drive) icon. Select "Applications" and finally double click "Disk Utility" under "Applications."
Select the disk from the list of hard drive(s) and disks on the left. There should be a small disk icon and then the name of the disk; it appears under a small disk icon with the name of the CD/DVD drive.
Choose "New Image" from the icons at the top of the window once the disk name is highlighted. It should appear as a drive icon within a page icon with a green and white plus sing in the top right corner.
Create a .CDR file by choosing "DVD/CD Master" from the Image Format selections. Give the file the desired name in the "Save As:" panel. Any other selection will create a .DMG file.
Convert a .CDR file to an .ISO file simply by clicking on the file name twice (not double clicking) so that the name is highlighted with a cursor. Erase CDR, and type "ISO." Press "Enter," and the file is ready for use by Windows-based computers.