Drive Letters and Partitioning your Hard Disk

Most people running a DOS/Windows or Windows NT system have their entire hard drive defined as C: (which is the way that the operating system installs by default). This can be illustrated by the following diagram.

single primary partition

I know that this doesn't look like much but please bear with me because I am about to explain how you might partition your hard drive to make it easier to backup your data and to enable you to run multiple operating systems.

The C: drive is actually a primary partition and it does not need to take up all of the space on your hard drive. You can have up to four primary partitions on your hard drive or three primary partitions and one extended partition. Extended partitions can in turn contain one or more logical partitions.

In the diagrams which follow, I will use divisions of the main box as shown above to represent the hard disk and dividing this into sections will represent dividing the drive into separate primary partitions, a second box within one of these divisions will represent an extended partition and divisions of this inner box to represent logical partitions. After discussing what partitions you might want to create and how DOS/Windows will assign drive letters to them, I will then move on to how you can create the partitions.

Logical partitions each get their own drive letter so partitioning your hard drive with one primary partition and one logical partition gives you the following arrangement where your hard drive appears to be two separate drives each with their own drive letter:

one each of primary and extended/logical partitions

We can split the extended partition into several logical partitions each of which will be allocated their own drive letter. This would enable us to install just the operating system and associated files on the primary partition (C: drive), put our application programs on a separate logical partition (D: drive), our data on yet another logical partition (E: drive), set aside a logical partition as a workspace eg. for storing files that are created during an application install (F: drive), and perhaps yet another logical partition to keep archived copies of our data (G: drive). Doing this can drastically reduce the rate at which your drive becomes fragmented when programs and data are mixed together on the same drive.

We also have a further two primary partitions that we can define. The primary partition that the computer boots from is called the active primary partition. The other primary partitions are actually hidden from the DOS/Windows operating system. The operating system treats them as if they don't exist. So what use are they? We can actually use them to store different operating systems eg. You might have Windows 98 SE in one primary partition and Windows ME in another. You can then use a product called a boot manager to select which of the primary partitions will be the active one and which will be hidden. This would enable you to test out your hardware and applications with Windows ME without affecting your working Windows 98 SE setup. Whichever primary partition is active will be the C: drive.

Implementing these gives us the following:

many primary and extended/logical partitions

So what does adding a second physical hard disk do to our drive lettering? Drive letters are allocated to the active primary partition on each hard drive in order before allocating letters to logical partitions. So if we add a second drive with one primary and one extended/logical partition we get the following:

first drive - changed drive letters
second drive with primary

If the second drive is in place when we first set up the system then everything is okay, the drive letters on the logical partitions are different than in the above example but as this setup will exist before we install anything everything will install okay.

If we add a second hard disk like this to an existing system then we have a problem because the drive letters have all changed and the operating system wont like it. Rather than reinstall all of our software to fix the drive letter problem we should instead define only extended/logical partitions on the new hard disk. Without a primary partition to claim a drive letter before the logical partitions on the existing drive the existing drive letters will be unchanged by the addition of the new disk.

first drive - unchanged drive letters
second drive without primary

So how do you partition your hard drive? The method provided by Microsoft that comes with your operating system is called fdisk. This program allows you to create and delete one primary partition and one extended partition (which can contain several logical partitions) on each hard disk. fdisk will not allow you to create multiple primary partitions on a disk. fdisk destroys all of the data on your partition when you delete it to create several smaller ones in the space (because fdisk cannot move or resize existing partitions) therefore if using this program to repartition the hard disk containing your operating system you will need to run it from your operating system boot disk and then reinstall everything.

So how can we repartition without losing our existing data? Well I use a product that I purchased from Powerquest called Partition Magic. This program can not only dynamically resize and move your partitions for you without losing your data and create more than one primary partition on your disk, it can move programs between drives making the necessary changes to the registry so that they will still work and it comes with a boot manager (called Boot Magic) that allows you to select which partition to boot from. The major disadvantage to this product is that it costs money. You have to buy it.

There are also free partition managers that you can download from the internet which are more powerful than fdisk although not as user friendly as Partition Magic. One of these is Ranish Partition Manager (which also has its own boot manager).

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