Windows A: and B: Drives Explained

Understanding Drive Letters in Windows

When installing Windows, the C: drive is automatically designated as the primary location. If your computer includes a CD/DVD drive, it is typically assigned the D: drive letter.

Any further storage devices connected to your system are then sequentially assigned drive letters following this pattern.

The Mystery of A: and B: Drives

This raises a common question: what is the purpose of the A: and B: drives? These drive letters are historically associated with older storage technologies.

Originally, the A: drive was reserved for the first floppy disk drive, and the B: drive for the second.

Even if your computer doesn't have a floppy disk drive, these drive letters are still often reserved by the operating system.

Origin of the Question

This explanation stems from a question posed on SuperUser, a question-and-answer platform that is part of the larger Stack Exchange network.

Stack Exchange is a collaborative collection of websites dedicated to providing answers to user-submitted questions.

The image accompanying the original discussion was created by Michael Holley.

A Historical Inquiry into Drive Letters

For those with a longer history in computing, the answer to the whereabouts of the A: and B: drives is immediately clear. However, more recent computer users may find these drive designations curiously absent from their systems.

A SuperUser forum member, Linker3000, recently inquired about this very issue:

Within Windows operating systems, the primary hard drive is consistently designated as C:. Subsequent drives, optical media, and removable storage devices are then assigned letters in sequential order.

Therefore, a second hard drive would typically be D:, a DVD drive E:, a USB drive F:, and so on.

This leads to the question: what happened to the A: and B: drive designations, and where were they located?

The mystery of the missing drive letters has been solved thanks to the insights of experienced computer users.

The Origins of A: and B:

Historically, the A: and B: drives were reserved for floppy disk drives. These were the standard method for portable data storage and software distribution prior to the widespread adoption of other technologies.

The A: drive was typically assigned to the first floppy disk drive, while B: was designated for the second. This allowed users to easily switch between disks without needing to remember specific drive paths.

The Decline of Floppy Disks

As technologies like CD-ROMs, USB drives, and eventually solid-state drives (SSDs) became prevalent, the need for floppy disk drives diminished. Consequently, the A: and B: drive designations became less common.

Modern computers often do not even include a floppy disk drive, meaning the A: and B: designations simply aren't assigned. The operating system automatically assigns letters starting with C: for the primary hard drive and continues sequentially from there.

Legacy and Virtual Drives

While rarely seen on modern systems, the A: drive designation can sometimes be found in older software or virtual machine configurations. It may also appear if a USB floppy drive is connected to the computer.

In these cases, the operating system will recognize the floppy drive and assign it the A: designation, reverting to the historical convention.

Understanding Drive Letters in Windows

The origins of drive letter assignments in Windows trace back to the earliest days of personal computing. Initially, computers relied solely on floppy disk drives for storage.

Early Computing and Floppy Drives

In the beginning, systems lacked hard drives entirely. A single floppy drive was standard, and adding a second represented a significant investment. Booting involved using one disk, then swapping to another for programs and data. This process often required repeatedly inserting and removing disks, prompted by messages like "Please insert source disk into drive A..."

The Introduction of Hard Drives

As hard drives became more affordable, systems commonly featured two floppy drives – one for booting and common programs, and another for data and specialized applications. Motherboard hardware consistently supported these two floppy drives at predetermined system addresses.

Consequently, the operating system adopted this hardware-based requirement, assigning the first hard drive the letter C:, and subsequent drives letters like D:, E:, and so on.

Transition to 3.5" Disks and Legacy Support

The shift from 5.25" to 3.5" floppy disks often saw both drive types present in a single system. This configuration was also supported by the motherboard hardware and the OS.

Because drive letters were rarely exhausted, the ability to reassign them wasn't considered crucial until the advent of plug-and-play standards, which abstracted drive addresses.

Backwards Compatibility and Current Practices

A substantial amount of software was developed with the expectation of finding long-term storage on the C: drive, including the BIOS itself. It remains possible to connect two floppy drives and operate a system like those of the early 1990s, utilizing drives A and B.

The primary reason for starting hard drive assignments at C: is therefore historical – maintaining backwards compatibility. Despite advancements in data storage abstraction, the OS continues to treat drives A and B uniquely, allowing for their removal without disrupting the OS, employing different caching methods, and exercising greater caution regarding their boot sectors due to early virus threats.

Drive Letter Restrictions and Indexing

Interestingly, Windows imposes restrictions on assigning drive letters A and B to hard drives. As one SuperUser contributor discovered, assigning these letters can prevent Windows from indexing the drive.

Microsoft documentation confirms that while A and B are typically reserved for floppy drives, they can be assigned to volumes if floppy drives are absent. However, doing so can lead to functionality issues, such as search indexing failures.

Attempting to use A or B for a primary drive may result in indexing problems, highlighting the ongoing legacy of the original drive letter conventions.

While assigning a data drive to A: or B: is possible, it's not recommended for boot drives.

Do you have additional insights to share regarding this topic? Please contribute your thoughts in the comments section below. For a more comprehensive discussion, explore the original Stack Exchange thread here.