I recently had an interesting conversation with a newly minted network administrator. We were discussing a problem I encountered at a client site regarding a missing drive letter. I was quite pleased with myself because I figured out what was wrong with the machine, and I shared my jubilance about being the “master of all things computing.”

The net admin responded with, “That’s interesting,” in a less-than-enthusiastic tone and chided me that hardware problems were the province of A+ techies. His opinion was that enterprise administrators do not need to understand “simple” hardware and software problems that could cause something like a missing drive. I disagreed with him and explained that an A+ techie might not be available at 3:00 A.M. on a Saturday night to solve these “simple” problems for him. He shrugged his shoulders and reluctantly agreed.

Missing drive letters are one of the less common, but more bedeviling troubleshooting issues you’ll experience. Breaking the problem down into the most likely culprits, however, will allow you to resolve the issue. I will take a closer look at some of the more common reasons (both hardware- and software-related) why drive letters might disappear and give you some tips on how to solve the problem.

Hardware problems
Most network administrators look for software configuration issues first and don’t consider the possibility of a hardware failure. As a network administrator, you should always start with layer 1, the physical (hardware) layer, and work your way up the OSI model when troubleshooting problems.

Common hardware-related issues that cause drive letters to be missing include:

  • Disk jumper settings
  • Cable problems
  • SCSI chain settings


An important clue that you’re dealing with a hardware problem is that all the drive letters associated with a particular physical disk are missing. This is because most hardware problems will cause the entire disk not to be recognized by the BIOS, so the operating system is completely unaware that the disk is there.

Drive jumper settings
An incorrect jumper setting is one of the most common hardware problems related to missing drive letters. New hard disks are typically set to make the drive a master. If the jumper settings are not changed and you add the disk to an IDE cable that already has a disk set as master, the disk may not appear to the BIOS (and thus it is missing when you boot up). Even more intriguing, I’ve seen incorrect jumper settings cause a disk to be recognized only intermittently at boot up.

To change the drive jumper settings, you’ll likely need to open the case and unscrew the drive from the housing. Or, you could also use a pair of long-handled needlenose pliers to change the jumper settings while the drive is still in place.

Another jumper setting to watch out for is the Cable Select setting. When both disks are set for Cable Select, the position of the disks on the IDE cable determines the master/slave relationship of the disks. If you choose the Cable Select option, make sure that both disks are configured for Cable Select. Be aware that some hard disks do not work with this option, and cable problems sometime prevent the Cable Select option from working correctly. In general, you should manually configure your disks as either master or slave.

Cable problems
A wide range of cable problems can cause entire disks (and their associated drive letters) to disappear, including the following:

  • Cable detached from hard disk
  • Cable detached from motherboard
  • Cable wires bent or broken
  • Cable reversed at motherboard or disk
  • Wrong cable type

Reseating the cable connections should always be your first step when troubleshooting missing drives. You won’t often see the cable completely detached from the motherboard or drive, but it may come loose over time due to subtle vibrations within the computer caused by a process known as socket creep.

Older cables can also wear out with age. I’ve seen older machines where the cables were tied up tightly for several years. The tight bends in the cable led to slight breaks in the cable wires. You can often tell if this type of subtle destruction of the cable is taking place because the disk will act erratically for a while before it completely disappears. There will be no obvious external signs that the cable is bad, so the only way to test for this problem is to swap out the old cable with a new one.

Another common reason that a disk won’t show up is when its cable is put on backwards. Always make sure that the red stripe (indicating Pin 1) on the cable is closest to the power supply. You may need a flashlight and/or magnifying glass to discover the marking on the motherboard connector that indicates which end is Pin 1.


If you just make sure that the cable is oriented in the same direction as another cable that’s working properly, you’ll probably be in good shape. If you don’t have a working cable connection to compare it to, however, you can check out the motherboard manual for instructions on how to orient the stripe on the drive cable at the motherboard connection.

The wrong cable type can also cause disk recognition problems. Newer UDMA 66+ disks require UDMA 66+ cables, so if you use a cable designed for older disks, your UDMA 66+ disks may not appear to the system.

SCSI settings
Some SCSI specifications support an auto-detect and configuration feature that automatically sets the SCSI ID settings on disks in a SCSI chain. The SCSI ID is a disk identifier that notes the unique position of each disk in a SCSI chain in the same way that the master/slave setting on IDE disks determines the positions of the disks on an IDE cable. If your SCSI disks and controller do not support the auto-configuration feature (supported by some SCSI III implementations), you’ll need to manually set the SCSI ID on each disk.

You should check the ID setting on each disk on the chain. The method of setting the ID varies with the disk and controller type. After confirming that the disk’s IDs are set correctly, check for SCSI chain termination. The controller should be self-terminating, but the last device on the chain must be manually terminated.


SCSI disks typically ship with a terminator already placed on the drive. Make sure that only the last drive on the chain has the terminator in position.

Software problems
Fortunately, there are a limited number of software problems that can cause a missing drive letter. In newer Windows OSs, such as 2000 and XP, you will rarely see a missing drive letter due to operating-system problems—but don’t let the modernity of your OS give you a false sense of security. Here is a short list of some of the more common software problems that can lead to a missing drive letter:

  • Disk compression schemes
  • Tweak UI
  • Partition Magic
  • BIOS configuration
  • Hot-swap floppy (Q271233)

Disk compression schemes
Disk compression schemes are a major offender when it comes to missing drive letters. Fortunately, the missing drive letter doesn’t lead to any practical problems with missing resources on the computer.

Disk compression schemes like DriveSpace3 (packaged with Windows 98) can cause a drive letter that was once there to no long appear. For example, suppose your computer has drives C:, D:, E:, and F:. DriveSpace3 gives you the option to compress an entire drive. If you decided to compress the entirety of drive D:, a new drive letter would appear (by default, this would be drive H:, although you can use any drive letter you want). Drive letter H: is actually the old drive D:, insofar as the files it holds are concerned, and the new drive D: represents the contents of a compressed file that is stored on the old drive D:.

The drive can end up missing if you (or someone else) select the option to Hide Host Drive (see Figure A). After the host drive is hidden, it will no longer appear in the My Computer or Windows Explorer applications. However, because the host drive doesn’t contain any usable free space, you won’t lose any computer resources. Microsoft provides the option to hide the host drive so that users will not make the mistake of trying to store information on the host drive.

Figure A
Hiding the compressed host drive

Another disk-compression-related problem lies with the old DoubleSpace application included with DOS 6.x. You may see the following error:
There are no more drive letters reserved for dblspace to use. To add more, use the options selection from the tools menu.

This error is seen when the DBLSPACE.ini is missing or corrupted, or when a user inserts a floppy disk that has been compressed with DoubleSpace on a machine on which DoubleSpace isn’t installed. (For more details on this error, check out Q93582 in the Microsoft database.)

Tweak UI
Tweak UI is a cool tools package from Microsoft that allows you to manipulate the desktop environment without having to root around in the Registry. Tweak UI allows you to manipulate the drive letters appearing in My Computer and Windows Explorer. If you remove the check mark next to a drive letter (see Figure B), that drive will no long appear to the user.

Figure B
Tweak UI configured to hide drive D:

This is a common problem among Tweak UI users. They often download the application and start messing with the settings. Then they notice at a later time that the system isn’t acting quite right. The most likely culprit in this case is Tweak UI. Whenever you troubleshoot a strange problem like missing drive letters, always check the Control Panel to see if Tweak UI has been installed. To restore the drive letter so it appears in My Computer and Explorer, just check its corresponding check box.

Partition Magic
Partition Magic is a popular tool for creating and manipulating drives and partitions. However, Partition Magic also allows you to get around basic DOS limitations, which can result in unexpected problems. For example, you can use Partition Magic to create multiple primary partitions on a DOS-based (or Win9x/Me) computer. However, when you boot into the operating system, only one of the primary partitions will appear, in spite of the fact that the drive letter appeared in the Partition Magic application.

The reason for this is that DOS only recognizes a single primary partition. All other partitions must be logical drives in an Extended Partition. The solution to this is to use Partition Magic to change the drive type to Extended, and then create logical drives inside the Extended Partition.

BIOS Configuration
The most vexing disappearing drive problems you’ll run into are the result of BIOS issues. Most BIOS vendors allow new drives to be detected and installed automatically. However, some vendors require that you enable the drive in the BIOS manually before you can use a new drive. Many of the popular Dell computers, such as the Precision and Dimension lines, ship with a BIOS that requires you to manually configure it in order for the drive to appear to the operating system.

Figure C shows an example BIOS configuration display. If the drive is configured as [None], the drive may not be detected. To make the drive appear to the system, you must configure the setting to Auto, or to a user-defined drive parameter. The Auto setting will work for virtually all systems created in the last 2-3 years.

Figure C
Configuring the BIOS to recognize a drive

Hot-swappable floppy drive
If you are working with a Windows Me machine, you might find that floppy drives mysteriously disappear. This happens when you use an external floppy drive that isn’t connected when the computer is turned on or while the computer is attached to a docking station. Check out Q271233 in the Microsoft database for details on a solution to this problem.

DOS LASTDRV entry in config.sys
If you are still supporting computers running MS-DOS, you need to be aware of the LASTDRV directive in the config.sys file, which instructs DOS to limit the number of drive letters based on the LASTDRV entry. If this setting is something like LASTDRV=E (which allows only three hard disk partitions, because A and B are reserved for floppy drives) and you create a fourth drive on the machine, the operating system will not assign a drive letter to that new partition or logical drive This problem doesn’t affect newer Windows operating systems, but it could wreak havoc with your DOS systems (including Windows 3.x running on DOS).

Missing drive letters are often due to hardware errors, and general troubleshooting procedures dictate that you start at the physical layer and seek these out first. If you can’t identify a hardware error, however, there are also some common software configuration problems that could be the culprit. Taking a methodical approach to troubleshooting missing drives should ensure that you will find the cause of the problem quickly and solve it in no time.