By Graham K. Rogers

Many panels in System Preferences have seen changes with the update to OS X 10.8, Mountain Lion. Startup Disk preferences appears to have remained mostly unchanged on the surface although there is one minor difference in the single pane.

The Startup Disk preference panel allows users to change the disk that is used to start the computer, to ensure that the correct disk or partition is being used, and also to restart the computer in Target Mode: as a slave disk. This is useful in analysis and repair. Target Mode can also be used for swift transfers of large amounts of data.

Startup Disks

If a disk has more than one partition, only those with operating systems installed will be shown. External, bootable disks will also appear in the panel when attached. In the screen shot I am using (above), there are five disks shown (left to right): the normal disk installed in the Mac with the current version of OS X; a Disk Warrior DVD inserted into the optical drive; an installer for OS X 10.7 on an external disk; an installation of Mountain Lion on a 16 GB flash drive; and one partition of a an external 500 GB drive attached via a USB port.

The "current disk" icon (below) also displays data concerning the version of the operating system in use and the disk name when highlighted. This should be done when switching startup disks to ensure the correct disk is being used. It may be better for the disk to be highlighted in normal working to ensure that there are no delays on a restart. OS X is usually good at finding a bootable disk, but has been known to lose the plot.

In this part of the panel -- to the right -- there is also a button marked restart. If a disk is inserted in the drive (like Disk Warrior), this can be selected instead of the main disk, and we can restart directly using the preference pane. When such an optical disk is used, we can also start the computer using the C key.

Any disk with a usable operating system can be selected in this panel and the computer will restart from that. If the computer cannot be started in order to make such a selection, it may also be possible to find a bootable disk by restarting with the Option key pressed. I have more information on Startup Keys in an earlier article available online.

Target Mode

Target disk mode can help in diagnostics and repair. As an example I can run Disk Warrior on my MacBookPro to check and repair the disk on my iMac. It is also a method to transfer large quantities of data quite quickly.

The computer will start in Target disk mode when the button (bottom right) is pressed. The target Mac should be connected to a (master) computer before using Target mode. Target mode may also be activated by starting a computer with the T key pressed. The screen on a Mac being used in Target mode will display a large Firewire icon while used in this way.

At the bottom of the Startup Disk panel, is a lock icon which requires an Admin password to open. This makes it less easy for someone to start the computer using an external disk. If used in conjunction with the Firmware Password Utility, the computer is considerably more secure. With Mountain Lion this feature may be found in the repair utilities accessed by starting the computer using the Command + R keys.

Graham K. Rogers teaches at the Faculty of Engineering, Mahidol University in Thailand. He wrote in the Bangkok Post, Database supplement on IT subjects. For the last seven years of Database he wrote a column on Apple and Macs.