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Removable Drive Letter Assignments

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Confused by the seemingly arbitrary way in which Windows assigns letters to your hard drive? You're not alone. This document explains the madness and helps you take control of how driver letters are assigned to your removable drives.

Removable Drive Letter Assignments

One problem people have when installing new drives is confusion with drive letter assignments. This becomes especially true when adding a new drive moves the assignments of previous drives, something that most people don't expect. Some simple rules govern drive letter assignments in Windows and DOS. These basic rules apply to all versions of Windows through Win98, Millennium Edition and NT, Windows 2000, and DOS, although in some operating systems and with some drivers you can have specific control over certain drives.

The basic rule is that devices supported by ROM BIOS–based drivers come first, and those assigned by disk-loaded drivers come second. Because floppy drives and hard drives are normally ROM BIOS supported, these would come first, before any other removable drives.

The system therefore assigns the drive letter A to the first physical floppy drive. If a second physical floppy drive is present, it is assigned drive letter B. If there is no second floppy drive, the system automatically reserves B: as a logical drive representation of the same physical drive A:. This allows files to be copied from one disk to another by specifying COPY file.ext A: B:.

The system then checks for installed hard drives and begins by assigning C: to the master partition on the first drive. If you have only one hard disk, any extended partitions on that drive are read, and any volumes in them are assigned consecutive letters after C:. For example, if you have a hard disk with a primary partition as C: and an extended partition divided into two logical volumes, they will be assigned D: and E:.

After the hard drive partitions and logical volumes are assigned, the system begins assigning letters to devices that are driver controlled such as CD-ROM drives, PCMCIA attached devices, parallel port devices, SCSI devices, and so on.

Here is how it works with only one hard drive split into three volumes, and a CD-ROM drive:

One Drive Primary Partition


One Drive Extended Partition 1st Volume


One Drive Extended Partition 2nd Volume


CD-ROM Drive


When a removable drive is added to this mix, it is assigned either F: or G: depending on the driver and when it is loaded. If the CD-ROM driver is loaded first, the removable drive is G:. If the removable drive driver is loaded first, it becomes F: and the CD-ROM drive is bumped to G:. In DOS, you control the driver load order by rearranging the DEVICE= statements in the CONFIG.SYS file. This doesn't work in Windows because Windows 9x, Millennium, NT, and 2000 use 32-bit drivers, which aren't loaded via CONFIG.SYS. You can exert control over the drive letters in Windows by manually assigning drive letters to the CD-ROM or removable drives. You do this as follows:

  1. Right-click My Computer, and choose Properties.
  2. Choose the Device Manager tab.
  3. Click the + next to the CD-ROM drive icon. Right-click the CD-ROM drive, choose Properties, and select the Settings tab.
  4. Select and change the Start Drive Letter.
  5. Select the same letter for End Drive Letter.
  6. Click OK and allow your system to reboot for changes to take effect.
  7. Repeat the previous steps by clicking the + next to Disk Drives and assign a different drive letter to your removable drive.

Using these steps you can interchange the removable drive with the CD-ROM drive, but you cannot set either type to a drive letter below any of your existing floppy or hard drives.

So far this seems just as everybody would expect, but from here forward is where it can get strange. The rule is that the system always assigns a drive letter to all primary partitions first, and all logical volumes in extended partitions second. This means that if you have a second hard disk, it also has primary and extended partitions, and the extended partition has two logical volumes, the primary partition on the second drive becomes D:, with the extended partition logical volumes becoming G: and H:.

Here is how it works with two hard drives, each split into three volumes, and a CD-ROM drive:

First Drive Primary Partition


Second Drive Primary Partition


First Drive Extended Partition 1st Volume


First Drive Extended Partition 2nd Volume


Second Drive Extended Partition 1st Volume


Second Drive Extended Partition 2nd Volume


CD-ROM Drive


In this example if a removable drive were added it would become either I: or J:. Using the same procedure outlined previously, you could swap I: or J: or assign them higher (but not lower) letters as well. Some factory-installed removable drives that use the IDE interface (such as some versions of the Zip 100 IDE) act like a second hard disk, rather than an add-on removable drive.

The utility software provided with the Iomega Zip and Jaz drives can also be used to assign the drive to any available drive letter.


Although some utilities are available for remapping drive letters under Windows, I normally don't recommend them. This is because if you boot under DOS or via a floppy, these remappings will no longer be in place and the standard BIOS mapping will prevail. Since one often boots to DOS to do setup, diagnostics, configuration, or formatting/partitioning, confusion about which drive letters are what can lead to mistakes and lost data!

If you reset your CD-ROM or other removable-media drive to a different drive letter in Windows, be sure to set the drive to the same letter when you run the computer in command-prompt or MS-DOS mode. This is typically done through command-line options you can add to the AUTOEXEC.BAT statement for the drive. See your drive's instruction manual for details.

About the Author

Scott Mueller has sold more than two million copies of his best-seller Upgrading and Repairing PCs since it became an instant classic in 1988. Scott's industry-defining hardware book has been translated into 11 languages and has received accolades from PC technicians, enthusiasts and students worldwide. Scott is president of Mueller Technical Research, an international research and corporate training firm. Since 1982, MTR has specialized in the industry's longest running, most in-depth, accurate and effective corporate PC hardware and technical training seminars, maintaining a client list that includes Fortune 500 companies, the U.S. and foreign governments, major software and hardware corporations, as well as PC enthusiasts and entrepreneurs. His seminars have been presented to thousands of PC support professionals throughout the world. Scott has developed and presented training courses in all areas of PC hardware and software. He is an expert in PC hardware, operating systems, and data-recovery techniques. For more information about a custom PC hardware or data recovery training seminar for your organization, contact Lynn at

Mueller Technical Research
21 Spring Lane
Barrington Hills, IL 60010-9009
(847) 854-6794
(847) 854-6795 Fax
Email: scottmueller@compuserve.com
Web: www.upgradingandrepairingpcs.com

If you have questions about PC hardware, suggestions for the next version of the book, or any comments in general, send them to Scott via email at scottmueller@compuserve.com. When he is not working on PC-related books or teaching seminars, Scott can usually be found in the garage working on performance projects. This year a Harley Road King with a Twin-Cam 95ci Stage III engine continues as the main project (it's amazing how something with only two wheels can consume so much time and money ), along with a modified 5.7L '94 Impala SS and a 5.9L Grand Cherokee (hotrod SUV).

About this Article

This article was derived from Upgrading and Repairing PCs, 12th Edition, by Scott Mueller. Published by Que (August 2000).

© Copyright Macmillan USA. All rights reserved.

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