The hard drive or solid state drive on your PC is called drive c, which implies the existence of a drive A and B. So what are drives A and B? What does the C drive stand for? Why is the hard drive called C?
Drive letters are a holdover from early microcomputers. The letters A and B went to floppy drives, so if you had a hard drive, it went to the earliest available drive letter, which was C.
What are drives A and B?
The concept of assigning letters to disk drives originated on IBM mainframes, specifically an operating system called CP/CMS from 1967. In the 1970s, when Gary Kildall was writing CP/M, the first popular operating system for microcomputers, he borrowed concepts from existing mainframe and mini computer operating systems. Some of the commands came from DEC mini computers. But he borrowed drive letters and his favored programming language from IBM mainframes.
In CP/M, the floppy drive was drive A. If you had a second floppy drive, it was drive B. If you didn’t have a second floppy drive, you had a virtual drive b, which you accessed by swapping disks. The system prompted you to insert a disk for drive B, and then insert a disk for drive A when it needed to access the A drive again.
If you had two drives, which drive was which depended on how the jumpers were set, or the drive’s position on the cable. Yes, there were standards, but not everyone followed them. Other operating systems did things differently. The operating system for Radio Shack TRS-80s used numbers, as did Commodore.
The IBM PC famously did not run CP/M, but the IBM PCs operating system, PC DOS, borrowed very heavily from CP/M, including the concept of drive letters and the assignment of them, most of the commands, and a lot of the internals.
What does the C drive stand for?
The drive letters are not an abbreviation for anything. They just follow in sequence. Early PCs did not have hard drives, because in 1981 the hard drive cost as much as the rest of the computer, but if you got a hard drive, it took the first available drive letter, which was drive C.
When floppy drives went obsolete is a matter of debate. Apple was the first company to decide they were obsolete and start leaving them out in 1998, but it took a few years for PCs to do the same. And this was controversial at the time. I was doing desktop support against my will in the late 90s, largely for Macs. Let me assure you, every one of those blue translucent Macs I worked on had a USB floppy drive hanging off the side.
For backward compatibility and interoperability purposes, if you plug a USB floppy drive into a PC, it took drive A. Moving the hard drive up to drive A would have introduced compatibility issues. Well behaved software will adapt. But a surprising amount of software makes certain assumptions about where it will be installed, including some very popular programs, like Adobe creative apps.
Windows is no longer based on MS-DOS, but it inherited drive letters from it, also for backward compatibility purposes.
That means that drive A, and especially drive B, are now vestigial organs. But momentum is a powerful thing. So that is why the hard drive is called C.
What about drive D?
And yes, if you are wondering, you can go all the way out to drive Z.When you got a CD-ROM drive, it took the first available drive letter by default. And you could assign it a different letter if you wanted.
If you had multiple hard drives, they took the next available drive letter. And if your hard drive was too large, you could split it into virtual drives and assign them multiple drive letters. They typically went in order.
David Farquhar is a computer security professional, entrepreneur, and author. He started his career as a part-time computer technician in 1994, worked his way up to system administrator by 1997, and has specialized in vulnerability management since 2013. He invests in real estate on the side and his hobbies include O gauge trains, baseball cards, and retro computers and video games. A University of Missouri graduate, he holds CISSP and Security+ certifications. He lives in St. Louis with his family.