Drive Letters

A peculiarity of the DOS world (and by extension, Windows 95/98) is that each drive in a computer (and each partition on a hard drive) is assigned a unique drive letter. Most people are familiar with A: as the floppy disk drive and C: as the primary hard drive. You may think you know what some of the other letters are used for, but the rest of them vary depending on your computer's configuration, and several of them may change when you add a new hard drive (or any type of drive for that matter.)

Drive B: is reserved for a second floppy disk drive. If your computer doesn't have two floppy drives, then B: is skipped. However, if you try to access drive B: from the DOS prompt, the computer will ask you to insert another disk into drive A: and it will pretend that it is actually reading a disk in drive B:. When you access drive A: again, the computer will ask you to put the original disk back in.

When you start MS-DOS (or Windows 95/98), the system automatically recognizes DOS partitions on hard drives which are configured in the computer's Setup utility. First it finds the primary DOS partition (or Pri-DOS) on each drive, and assigns consecutive letters to those. Therefore the primary DOS partition on the first hard drive gets the letter C:, and if you have a second hard drive with a primary DOS partition it will be assigned D:, and so forth. Next, if there are any extended DOS partitions on these drives, they will be assigned the next consecutive letters. One possible organization is shown in the following table:

After this point, things get a little bit more tricky. If you're using DoubleSpace or DriveSpace, DOS will create a new drive letter to access the compressed files. Instead of being consecutive with the other letters, however, it usually assigns a letter further down the line such as H: or I:. Right away, it then swaps the new letter with that of the uncompressed drive; so, for example, drive C: becomes the compressed files and drive H: becomes the uncompressed DOS partition on the drive.

This point marks the end of the drive letters that are assigned automatically. Any other drives not yet assigned a letter must use device drivers in the CONFIG.SYS and/or AUTOEXEC.BAT file (or Windows 95/98 itself.)

If additional drives are present that were not detected by the BIOS, there is probably a line in CONFIG.SYS which reads something like:

This defines the highest letter that can be used to reference additional drives. If this line is absent, DOS will use the minimum number of letters required for the drives already found (plus maybe one more.) The remaining drive letters are available to device drivers on a first-come, first-served basis. And unless told otherwise, they will usually take the drive letters in order. Therefore, a device driver in CONFIG.SYS (such as RAMDRIVE.SYS) will get a letter before one in AUTOEXEC.BAT (such as MSCDEX.EXE).

As a final note, the CD-ROM driver is split into two parts -- one in the CONFIG.SYS file and the other in AUTOEXEC.BAT. The latter is almost always MSCDEX.EXE, and that is the half which assigns the drive letter to the CD drive. If you wish to assign a particular letter, you should have a /L: switch on that line. For example:

Windows 95/98

Device drivers for Windows 95/98 don't require any drivers to load in CONFIG.SYS and AUTOEXEC.BAT, but if you do use both, you need to make sure they assign the same drive letter. If you only use the Windows 95 drivers, you can easily assign almost any drive letter using the Device Manager.

From the Control Panel, open the System icon, then go to Device Manager. Then select the drive you want to change the letter of, and click on Properties.

In the drive's Properties window, click on the Settings tab. At the bottom there will be a box that shows the current drive letter, and additional boxes where you can specify which letter you want the drive to have. There are two boxes, starting and ending, because certain types of drives can use more than one letter. In most cases, however, these two boxes should be the same.