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Partition types

There are three basic types of partition: Primary, Extended, and Logical. The types that you initially use to divide up a drive are Primary and Extended. Extended partitions can then be subdivided further into Logical partitions. Each hard drive can contain up to four Primary partitions, or up to three Primary partitions and and one Extended partition. When there are multiple primary partitions on a hard drive, only one can be active at any one time. The other primary partition(s) are hidden. They are physically on the disk, but cannot be "seen" by the OS in any way. The reason is that within the Master Boot Record (the first physcial sector of the hard drive) there is a partition table that tells the OS which is the active partition, and the boot process jumps to the active partition. Multiple primary partitions on a hard drive can be one way (though awkward) of making a multi-boot system. Frankly, I would never do this, and offhand I cannot think of why I would want to use more than one primary partition on a single drive, but it is worth knowing it can be done. I once solved a problem for a guy who could not figure out why his hard drive seemd to display only half of its real size. Turns out he had created two equal partitions, made them both primary partitions, and then of course could only see one partition at a time. I deleted the second primary partition, created an extended partition out that space, and he was in business.

An Extended partition can contain multiple Logical partitions, and they will all be visible at once if formatted in an appropriate file system. For simplicity, let's assume that all partitions in the remaining discussion are formatted as plain old FAT. That way we can focus on the partitioning issues without getting sidetracked into file system issues.

You could then have a drive with one Primary partition, one Extended partition, and have the extended partition divided into three Logical partitions. That would give you four partitions total, as follows:


graphic to illustrate partition divisions


Drive Letter Assignments

Assigning drive letters to these partitions is something that DOS and Windows (and OS/2, for anyone who is interested<g>) operating systems do. Other operating systems identify dirves in different ways. In the DOS/Windows/OS/2 world, the first two drive letters are reserved for floppy disk drives. The early DOS computers had twin floppy drives before they had hard drives, and this convention has stuck. The first "fixed disk" drive is always C:, therefore. If you have a hard drive that is formatted as one large partition, this may be the only hard drive you see. Then the CD-ROM is assigned the next open slot, D:, and maybe you have a Zip drive as E:. All very simple. But if you choose to partition your hard drive into multiple drives, it gets a little more complicated. The Primary partition (and remember that there can only be one Primary partition that is "active", or visible, on any hard drive at any time) gets its letter first, followed by any Logical partitions that are contained within an Extended partition. So if you partitioned the hard drive the way we illustrated above, your Primary partition would be the C: drive, the first Logical partition would be D:, the second Logical partition E:, and the last Logical partition F:. The CD-ROM would then be G:, the Zip drive H:, etc.

This is still not too difficult. But one day you discover that your hard drive is running out of space. You could buy a new one, even larger, but you have all of these programs and data on the existing disk, and you don't want to move all of that stuff, reinstall software, etc. So you decide to add second disk! Gets around all of those problems. If you create one large Extended partition on this disk, and create three Logical partitions within it, no big deal. Those three Logical partitions then become G:, H:, and I:. The CD-ROM becomes J:, and the Zip drive K:. But chances are you will have one problem to resolve. Since your CD-ROM drive has now changed its drive letter designation, any program that looks for a CD in the drive (like many game programs, or software that you run from the CD-ROM drive) will not be able to find it. There are software utilities that can solve that for you. (see end of article)

But, suppose you decided that you wanted to have a Primary partition on the second hard drive in addition to the one you have on the first hard drive. Why? Some operating systems can only boot from a Primary partition, and maybe you wanted to have a dual boot system using two of the operating systems. So suppose you have 2 hard drives, and each hard drive is partitioned as in the illustration above, i.e. one Primary partition, and three Logical partitions within an Extended partition. Total=8 partitions. In this case, the order of assigning drive letters gets a little more complicated. They get assigned in this order:

  1. The Primary partition from the first hard drive.
  2. The Primary partition from the second hard drive.
  3. The Logical partitions from the first hard drive.
  4. The Logical partitions from the second hard drive.

The general rule is that all Primary partitions get assigned first, then all Logical partitions. In our example, it looks like this:


Drive Letter Partition
C Primary partition, first hard drive
D Primary partition, second hard drive
E 1st Logical partition, first hard drive
F 2nd Logical partition, first hard drive
G 3rd Logical partition, first hard drive
H 1st Logical partition, second hard drive
I 2nd Logical partition, second hard drive
J 3rd Logical partition, second hard drive


Again, one result of this change is to scramble your software even more. What used to be the D: drive is now the E: drive, and any shortcuts to your software will now fail. You'll be getting "file not found" all over the place. There are three utilities I know of that can unscramble this mess. One comes bundled with the partitioning program Partition Magic, from Power Quest. The second is later versions of Norton Utilities, or Norton System Works, from Symantec. The third is a PC Magazine utility called COA32. If you know of any others, please let me know so I can update my records.


Other reading:

Neil Rubenking on FAT 32 and Drive Partitioning