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Booting From Alternate Drives

Overview

If serious system errors prevent you from even booting up your computer at all, being able to start the computer from a different hard drive can prove to be a useful troubleshooting resource. Both Mac OS X and Windows XP let you boot the system from an "alternate" hard drive.

Reader Jason Wiley describes the value to him of being able to boot from alternate drives:
"...this may be rare for you, but I am constantly using alternative boot drives for a variety of reasons, and most IT guys I know do as well. The main reason is for backups. Sure, I can backup to a DVD and spend time restoring my system if something bad happens, or I can use Carbon Copy Cloner, or do a mirror install to a FireWire drive or another ATA drive, or just drag and drop, and be up and running with a simple reboot. Also, I can have my very own system available to me at work without stepping on IT’s toes by loading software on their systems, thanks to a portable FW drive or iPod. If I want to change machines, it’s as easy as cloning the drive, and rebooting from that drive. On my web server, I have the internal ATA drive (in an original iMac) partitioned into 3 drives. The first is the main boot drive, the second, a copy that update every 1-2 weeks, and the third is a large volume for data only. If I ever have a problem with the web server, database (MySQL), scripts (PHP), etc, or any system level gremlins, then I am 1 reboot away from being up and running again while I fix the problem. On my main desktop machine, I do the same to an external FW drive. It’s all infinitely useful if you actually utilize the tool."

Mac OS X

OS X provides three methods for booting from an alternate drive:

  1. If the computer is already booted up and running OS X, you can select the alternate drive (as long as it contains a valid operating system) from the Startup Disk pane of System Preferences.

  2. Hold down a key (or a combination of keys) while booting. The following is a list of startup key combinations for booting from alternate drives (source: AppleCare Document 75459), which is handled by Open Firmware:

    3. Keyboard Shortcut System Action
    Option--Shift-Delete Bypass primary startup volume and seek a different startup volume (such as a CD or external disk)
    C Start up from a CD that has a system folder
    N Start up from a compatible network server (NetBoot)
    T Start up in FireWire Target Disk mode
    Option key Displays all system folders (OS 9 or OS X) on all connected volumes. Choose the one you want to boot, then click Continue.

  3. Use Apple's NetBoot to boot off of a remote server (see "A close-up look at Mac OS X's NetBoot," ComputerWorld).

Windows XP

Thanks to forum member UnnDunn for help with this section

Booting to an alternate drive is entirely controlled by the BIOS (Basic Input/Output System), which varies wildly from computer to computer. When the computer boots, it looks at different connected volumes in a certain order, called the "boot order." Most recent PCs place the CD-ROM or other such drives earlier in the boot order than the computer's hard disk, so if a CD is inserted, the computer will attempt to boot off of that first. If no additional media is inserted, the computer will skip over these other drives, and will boot off the hard disk like normal. While some computers require that you enter the BIOS setup to change the boot order, some let you press a hot key during startup to choose where to boot from.

XP's Product Activation may make it impossible to boot from an alternate IDE drive (other than NetBOOT).

Conclusion

Much of what is described in this section is not handled at the operating system level, but at a lower level. On the Mac, this lower level is called Open Firmware, while on the PC, it is called BIOS. Apple has the advantage here in that they manufacture both the computer hardware and the software that runs on that hardware. This lets them control this feature in its entirety, such that the process is identical on all Macs. Microsoft doesn't manufacture PCs, so it is impossible for Windows XP to offer any sort of consistent control over this feature — this is left up to the PC manufacturer (Dell, HP, etc.).

Even though Apple may have the advantage here in real world use, it would be unfair to assign OS X a higher score than Windows XP, due to the fact that this is an issue between the hardware and low-level system software. The operating systems have no role in this feature. Thus, this will remain a non-scoring section.

Non-scoring section

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