Why was "C:" chosen for the first hard drive partition?

Question

I was reading this answer, as the question came to my mind: why does C: indicate the first hard drive partition?

The usage dates back to CP/M (as noted in a comment), was embraced by MS-DOS, and persists in Windows 10. I looked around but could not find any better explanation than

The computer reserves A: and B: drive letters for the floppy disk drive and removable media, such as tape drives, even if these devices are not installed in the computer. As you install other drives and create new partitions, they are assigned to other drive letters after C, such as D, E, F, G, etc.

which I took from here and it's basically the same thing that is on Wikipedia and in the aforementioned SE answer.

I may be in for disappointment, but I wonder if there is more to the story. To draw a parallel, in many scientific fields (𝑥, 𝑦, 𝑧) is acknowledged as a tuple of unknowns. The rationale behind this choice is likely that the first letters of the alphabet were used for known quantities, and the last for unknown quantities (as per this answer on MathOverflow).

So the question may probably be better phrased as: did C: have a meaning which was unrelated to the existence of two floppy drives?

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UPDATES: It has been pointed out in a comment that a very similar question has also been asked, answered and massively upvoted on superuser SE. Does it even make sense at this point for this question to exist here? Mods and admins, feel free to close or take any action you deem appropriate!

Answer 1

TL;DR:

Why has “C:” been chosen for the first hard drive partition?

Because it is the first letter after A and B. Drives are simply numbered in sequence using letters. Since the vast majority of systems in use had only one or two floppy drives, C was usually the first number to be assigned to the next drive found after them.

Did C: have a meaning which was non-depending of the existence of two floppy drives?

No, drive letters have no meaning at all. They are simply variable names chosen in sequence of detection.

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In Detail

why does C: indicate the first hard drive partition?

Because it is the first letter after A and B, which were defined for floppy drives - as in the beginning, there were only floppies.

The fact that C is always the primary HD (or partition) is, as explained in the linked answer, a later mechanic, made to please (install) software that was made to assume C the main installation drive.

The usage dates back to CP/M (as noted in a comment), was embraced by MS-DOS, and persists in Windows 10. I looked around but could not find any better explanation than [A&B are fixed while further letters are assigned in sequence of drives 'discovered']

That's exactly the point. Internally drives (in CP/M) were simply numbered. The use of letters is a translation of this number into a letter. 1st (Drive) -> A, 2nd -> B and so on.

The sequence was not related to types of media or drives. A hard disk could well be drive A, while the first Floppy could be E, if being the 5th drive reported by BIOS. Similar there was no virtual drive B. Only available drives got numbered.

MS-DOS changed this by putting floppies first and adding a virtual floppy drive B for single floppy systems (yes, they were common, after all, 160 KiB is a lot of storage). This simplifies operation of software that needs two disks at once - not just for copying, but every other usage - like having a system disk with OS and application, for example a word processor, and another for data (read: text files). With only one drive a software had to handle disk change on it's own, not a nice task - especially if other components may be needed from the system disk. Handling two disks independently in the same drive did away with all issues (*1), software could assume there are always two drives to work with (*2).

So in an MS-DOS system C was always the first available drive number (thus letter) after the mandatory (at the time) floppy(s). When (later) hard disks became available they were usually mounted as C. Again (much) later when hard disk became a commodity, software started to assume C as being a hard disk, the same way they did for A&B as floppies. Which of course did not work out as easy with systems offering more floppy drives than two. So, again later (with DOS 5) C was finally reserved for the primary boot disk/partition (if not being a floppy).

which I took from here and it's basically the same thing that is on Wikipedia and in the aforementioned SE answer.

Finding it in many variations is usually(*3) a good hint that it really is the way it was made.

I may be in for disappointment, but I wonder if there is more to the story.

No, there is no meaning with drive letters. It's simply meant to number the drives, so they can be distinguished.

To draw a parallel, in many scientific fields (𝑥, 𝑦, 𝑧) is acknowledged as a tuple of unknowns. The criteria behind this choice is likely that the first letters of the alphabet were used for known quantities, and the last for unknown quantities (as per this answer on MathOverflow).

Well, that's exactly as here. The sequence of known drives starts with A and goes up the alphabet order.

Did C: have a meaning which was non-depending of the existence of two floppy drives?

No, drive letters have no meaning at all. They are simply variable names chosen in sequence of detection.

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Beyond One's Own Nose

Putting aside if meaningful drive names are a good thing or not, other systems did use more meaningful names, like Amiga-DOS used DFn for floppies (with n sequential numbered starting with Zero) and DHn for hard drives. The problem about locating the system drive was solved by adding logical names like SYS for the system drive/directory.

Then again, why use drive designators at all?

Like Unixoides, simply just work with their directory tree, having files (directories) from some various drives - and points thereof - simply mounted anywhere. Some not adding any way to access the drive as such, others offering symbolic entries to do so. Being somewhere in the middle between desktop and mainframe this dual handling makes quite sense.

And then there are systems, like all TSOS offspring, that do not care much about directories or alike but manage everything in a single catalogue database with volume names as attributes to the file entry. These being mainframe systems, the user does not have any idea about drives. For most parts not even drive types, or if it's on a disk or tape (*4) If at all, a volume were the file is located is known. In addition, drives do not have fixed hardware names - by exchanging a simple plug they can be renamed on the fly ...

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*1 - That is except the dreaded message to swap floppies.

*2 - Apple did go a step further by using volume names, so the single Mac drive could handle multiple disks concurrently, notifying the user to swap whatever was needed next . Which is as well less tricky in a windowed system allowing modal message boxes than on a line based interface.

*3 - Usually, as we all know one or more of those hard to kill myths being repeated over and over on the net-like Voyager running on 1802 CPUs.

*3 - Random reads on non cached tape files just happened to be somewhat ... err ... slow.

Answer 2

The letters A: and B: were reserved by MS-DOS, which was essentially a clone of CP/M and followed its usual practice, for floppy disk drives. If only one such drive was fitted, both letters were mapped to the same physical drive via prompts for the user to swap disks.

C: was simply the next available letter. Early filesystems had severe limits on how large a partition they could handle, so it was common for drives to have multiple partitions to collectively make use of an expensive hard disk's capacity, and these gained letters in sequence from there. The FAT16 filesystem in MS-DOS relieved these limits for some years, until hard disk capacity again caught up.

Drive M: was commonly used to refer to a temporary filesystem held in RAM. This was far enough ahead in the sequence that it didn't usually interfere with the above scheme.

When CD-ROMs became available, the driver installation scripts often also just picked the next available letter. This led to compatibility problems with some poorly-written game installers which assumed the user had a particular number of hard disk partitions, and hence a particular CD-ROM drive letter.

Network drives, where the physical storage was attached to a different computer entirely, usually avoided this situation by assigning letters late in the alphabet, such as T: or X:. Diskless workstations, however, could often be found running from a network drive mapped to C:. I used one such system while taking an office-computing class.

Answer 3

The bit that the previous answers are not emphasizing is that hard drives were unheard of. Computers came with floppy drives (if you were lucky...), more likely a cassette tape. So the numbering was logical, you booted from drive A: . If you were lucky to have a drive B: for data, then that was used, otherwise you took the program disk out of drive A: and put in your data disk.

There were no hard disks. Well there were - but these were luxury devices.

The first computer I programmed was my fathers TRS-80 that he ran his news-agency business from. All his customer data and accounting was stored on floppy disks, as well as all the software, and operating system.

You inserted your DOS Disk - there were a choice of flavors, including MS DOS, DR-DOS, PC-DOS etc. Then you inserted your expensive custom software program, then you inserted disks for storage when prompted.......

And now I feel old....