The Debug menu and other secret features in Disk Utility
Disk Utility in OS X on the Mac is a useful tool that most people are familiar with. It performs many functions and one of these is to display the disks drives and partitions available. You would think it would show everything, but it doesn't and in OS X Lion for example, it doesn't show the Recovery partition. What else is it hiding? Find out by enabling the secret Debug menu.
The Debug menu
This adds a new item to Disk Utility's preference file in the Library folder called DUDebugMenuEnabled and sets its value to 1, which means enabled. Now run Disk Utility in the Utilities folder and you will see a new Debug menu that wasn't there before.
Go to the Debug menu and click Show Every Partition - make sure it is ticked. Select the hard disk drive in Disk Utility's left panel and you will see more items than you used to. There is the boot partition plus one or two more in light gray. These light gray items are hidden partitions on the disk drive and the first one is called disk0s1 on my Mac, but it could be named something else on yours. It is a hidden system partition that controls the boot process and all Macs have it. Don't change anything on it.
If you have OS X Lion you will also see a hidden item called Recovery HD partition. This is the Recovery Disk that Lion creates when it is installed. Start the Mac with the Option key held down and all the boot disks and partitions are displayed. You can choose to start the Mac from the Recovery Disk, which can then be used to reinstall OS X, repair disk faults and so on. Your Mac will work fine without it, but it is obviously useful in case anything ever goes wrong with your Mac, so don't change it in any way.
You might like to check out a USB disk drive and see if there are any hidden partitions on it and explore the other Debug menu commands.
Advanced format conversion
OSX treats a disk image like a virtual disk drive and it appears in Finder and to apps like it was a real disk. It is possible to change the format of a disk image.
In Disk Utility, click File, Open Disk Image. Select a .dmg file (try your downloads folder, there are usually some in there) and open it. Select it in the left panel and then click Images, Convert. This is used to convert the format of a disk image from one type to another. Click Image Format and take a look at the options. The disk image can be converted to read-only, compressed, and , well, that's about it.
Quit Disk Utility, open a Terminal window again and enter the following command to enable advanced image options:
Start Disk Utility again, open and select a disk image, then click Images, Convert. Click Image Format and now there is a long list of disk formats that the .dmg file can be converted to. It could come in useful if you ever need to convert a disk image format. There are three different compressed formats, for example, and you may find that one compressed the contents better than another depending on the file types.