By Graham K. Rogers

Many panels in System Preferences have seen changes with the update to OS X to 10.9, Mavericks. The Startup Disk preferences panel appears to have remained unchanged. Startup Disk is used to specify the disk or system to start a Mac. This may need to be changed when maintenance is needed, or when using the computer in Target Mode: as a slave to another Mac.

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

Startup Disk Preferences: 15" MacBook Pro Mid-2010

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 taken from a 2010 MacBook Pro, I am using (above), there are four 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 installation of OS X 10.8 on an external disk; and an OS X 10.8.4 installation of Mountain Lion on a 16 GB flash drive.

In a second screen shot taken from the latest MacBook Pro (released November 2013) only three disks are shown. The newest Mac has no disk drive so Disk Warrior cannot be used as a startup disk in this way.

Startup Disk Preferences: 13" Retina MacBook Pro Late 2013

Note that, although the 10.8 installations are shown in the list of available startup disks, the November 2013 Mac cannot be started with these: a Mac can only be started with a version of OS X the same as (or released after) the original installation.

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.

When any disk is selected (and highlighted) a confirming text appears below: for example, You have selected OS X, 10.9 on the disk "Macintosh HD".

In this top part of the panel - to the right - there is also a button marked restart. For those computers with an optical drive, if a disk is inserted in the drive (like Disk Warrior), this can be selected instead of the main disk. We can restart directly using the preference pane or, when such an optical disk is used, by using the C key at startup.

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 icon while used in this way. With the older Macs this is a Firewire icon. The latest Macs show the Thunderbolt symbol when in Target mode.

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 and Mavericks the Firmware Password Utility feature may be found in the repair utilities accessed by starting the computer using the Command + R keys.

See Also:

• Accessibility
• App Store
• Bluetooth
• CDs & DVDs
• Date & Time
• Desktop & Screensaver
• Dictation & Speech
• Displays
• Displays
• Dock
• Energy Saver
• General
• iCloud
• Internet Accounts
• Keyboard
• Language & Region
• Mission Control
• Mouse
• Network
• Notifications
• Parental Controls
• Printer & Scanner
• Security & Privacy
• Sharing
• Sound
• Spotlight
• Trackpad

Graham K. Rogers teaches at the Faculty of Engineering, Mahidol University in Thailand where he is also Assistant Dean. 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.