Drive Letter Assignments | Scott Muellers Upgrading and Repairing Laptops, Second Edition

One problem people have when installing new drives is confusion with drive letter assignments. This becomes especially true when adding a new drive (such as in a swappable drive bay) moves the assignments of previous drives , something that most people don't expect. Some simple rules govern drive letter assignments in Windows and DOS.

All versions of Windows (as well as MS-DOS) treat floppy drives the same way, but major differences exist in the management of hard drives, CD-ROM/optical drives, and removable-media drives with Windows 9x/Me on the one hand and Windows NT/2000/XP on the other hand.

With any version of MS-DOS or Windows, the system 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 no second floppy drive exists, the system automatically reserves B: as a logical drive representation of the same physical drive A:. This allows you to copy files from one disk to another by specifying COPY file.ext A: B: .

MS-DOS and Windows 9x/Me Disk Management

The basic rule with these operating systems is that devices supported by ROM BIOSbased drivers come first, and those assigned by disk-loaded drivers come second. Because floppy drives and hard drives are normally ROM BIOS supported, these come first, before any other removable drives. After assigning A: and B: to the floppy drives, 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, PC Card/CardBus (PCMCIA) attached devices, USB or 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:

Device	Letter
One drive primary partition	C:
One drive extended partition, first volume	D:
One drive extended partition, second volume	E:
CD-ROM drive	F:

When a removable drive is added to the system proposed in the previous scenario, 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, Me, 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 with Windows 9x and Windows Me:

Right-click My Computer and select Properties.
Select the Device Manager tab.
Click the plus sign (+) next to the CD-ROM drive icon. Right-click the CD-ROM drive, select Properties, and select the Settings tab.
Select and change the Start Drive Letter setting.
Select the same letter for End Drive Letter.
Click OK and then allow your system to reboot for changes to take effect.
Repeat the previous steps by clicking the plus sign next to Disk Drives and assigning a different drive letter to your removable drive.

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

So far, this seems just as one 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 installed in a swappable drive bay, it also can have primary and extended partitionsthe extended partition can have two logical volumes, and the primary partition on the second drive could become 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:

Device	Letter
First drive primary partition	C:
Second drive primary partition	D:
First drive extended partition, first volume	E:
First drive extended partition, second volume	F:
Second drive extended partition, first volume	G:
Second drive extended partition, second volume	H:
CD-ROM drive	I:

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 both higher (but not lower) letters. Some factory-installed removable drives that use the ATA interface (such as some versions of the Zip 100 ATA) act like a second hard disk, rather than an add-on removable drive.

The utility software provided with removable-media drives such as the Iomega Zip and Jaz drives and others also can be used to assign the drive to any available drive letter.

To avoid scrambling existing hard drive letters when you install additional drives in a swappable drive bay, prepare additional hard drives with extended partitions only. Drive letters in the extended partition on the second drive will follow the drive letters in the first drive's extended partitions and so forth if you install a third or even more hard drives. Here's what the result would be when you have two hard drives, with the first drive split up as a primary disk partition with two drives in an extended partition, and the second drive having no primary partition but three logical drives in its extended partition:

Device	Letter
First drive primary partition	C:
First drive extended partition, first volume	D:
First drive extended partition, second volume	E:
Second drive extended partition, first volume	F:
Second drive extended partition, second volume	G:
Second drive extended partition, third volume	H:

Drive Remapping Utilities

Although some utilities are available for remapping drive letters under Windows, I normally don't recommend using them. This is because if you boot under DOS or via a floppy, these remappings are no longer in place and the standard BIOS mapping prevails. Because you might often boot to DOS to perform setup, diagnostics, configuration, or formatting/partitioning, confusion about which drive letters are which 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.

If you connect an external hard disk using the parallel port, SCSI, USB, PC Card, or IEEE 1394 interface to your laptop, it is treated as a removable-media drive and its drive letter is assigned after the drive letter(s) used by internal hard disks.

Windows NT/2000/XP Disk Management

Disk management for NT-based versions of Windows, including Windows NT/2000/XP, offers many more options than in MS-DOS and Windows 9x/Me. The Disk Administrator tool is launched from the Drive Properties Tools menu in Windows NT 4.0. The similar Disk Management tool is started from the Computer Management (Microsoft Management Console) utility in Windows 2000 and XP. Both Disk Administrator and Disk Management can be used to control drive letters. To change a drive letter with either tool, right-click the drive and select Change Drive Letter and Path .

Although C: is assigned to the first hard drive by default in these versions of Windows, you can assign the drive letters you prefer to both CD-ROM/removable-media drives and to logical drives on a hard disk. By default, when additional hard diskbased logical drives are added to a system, existing CD-ROM or other removable-media drive letters are not disturbed. Therefore, programs that depend on CD-ROM or removable-media drive paths continue to work.

You also can change the default boot drive letter from C: to another drive letter, but this is not recommended because it prevents your system from booting until manual Registry changes are made to correct the problem.

Note

If you create a dual-boot configuration with your laptop computer, the second operating system you install will use a different drive letter than C: for its default boot drive (even though they coexist on the same physical drive).