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Figure 12-2 The MBR checks the partition table to find the active partition.

The boot sector at the beginning of the hard drive isn’t the only special sector on a hard drive. The first sector of the first cylinder of each partition also has a special sector called the volume boot sector. Although the “main” boot sector defines the partitions, the volume boot sector stores information important to its partition, such as the location of the OS boot files. Figure 12-3 shows a hard drive with two partitions. The first partition’s volume boot sector contains information about the size of the partition and the code pointing to the boot files on this partition. The second volume boot sector contains information about the size of the partition.

Figure 12-3 Volume boot sector

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NOTE Every partition on a hard drive has a volume boot sector.

Primary Partitions

If you want to boot an operating system from a hard drive, that hard drive must have a primary partition. The MBR checks the partition table for the active primary partition (see Figure 12-4). In Windows, the primary partition is C:, and that cannot be changed.

Figure 12-4 The MBR checks the partition table to find a primary partition.

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NOTE Don’t confuse primary partition with primary controller. The latter, as you’ll recall from Chapter 11, “Hard Drive Technologies,” refers to the first PATA drive controller on a motherboard.

Even though hard drives support up to four primary partitions, you almost never see four partitions in the Windows world. Windows support up to four primary partitions on one drive, but how many people (other than nerdy CompTIA A+ people like you and me) really want to boot up more than one OS? We use a number of terms for this function, but dual-boot and multiboot are the most common. The system in my house, for example, uses four primary partitions, each holding one OS: Ubuntu Linux, Windows 2000, Windows XP, and Windows Vista. In other words, I chopped my drive up into four chunks and installed a different OS in each.

To do multiboot, most people use a free, Linux-based boot manager called GRUB (Grand Unified Boot Manager), although some people prefer a third-party tool such as System Commander 9 by VCOM to set up the partitions. Windows 2000 and up come with similar tools that can do this, but they can be messy to use, and GRUB helps simplify the process. When the computer boots, GRUB yanks control from the MBR and asks which OS you wish to boot (see Figure 12-5). You select an OS and it appears.

Figure 12-5 GRUB’s OS selection menu

Again, few systems use more than one primary partition. You may work on PCs for years and never see a system with more than one primary partition. The CompTIA A+ certification exams certainly don’t expect you to demonstrate how to create a system with multiple primary partitions, but they assume that you know you can add more than one primary partition to a hard drive if you so desire. The rest of this book assumes that you want only one primary partition.

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NOTE If you get the error “No Fixed Disks Present,” you can bet you forgot to tell the CMOS to look for the drive. Reboot, access CMOS, and try setting up the drive again.

Active Partition

When you create a primary partition and decide to place an OS on that partition, you must set that partition as active. You must do this even if you use only a single primary partition. Luckily, this step is automated in the Windows installation process. Consider this: When would you want to go though the steps to define a partition as active? That would be when you install an OS on that partition. So when you install Windows on a new system, the installation program automatically sets up your first primary partition as the active partition. It never actually says this in the installation, it just does it for you.

So if you raise your right hand and promise to use only Microsoft Windows and make only single primary partitions on your hard drives, odds are good you’ll