As far as one can remember in the history of computers, the hard disk drives on all personal computers which use Windows OS, the letter “C” has been used as the default hard drive. Now, let's look into the reason why “C” has been the favourite till now.
The idea to designate different storage devices by using simple letters is attributed to the IBM’s virtual machine operating systems developed back in the 1960s; starting with CP-40 and CP/CMS systems, and later, those that are copied by the CP/M operating system created by Digital Research, Inc. In the early operating systems (CP/CMS) the letters were used mainly for designated logical drives, even though later (such as with CP/M), they were mostly used to specify physical storage devices.
Then came 1980: when IBM tried to use the popular CP/M operating system on its IBM Personal Computer, talks between IBM and Digital Research, Inc went down the drains over some minor issues. The trouble allegedly started when Dorothy Kildall, the wife of CP/M inventor Gary Kildall, refused to put her signature on a non-disclosure agreement which IBM wanted.Thanks to the advice of Gerry Davis, Digital Research’s attorney, she made an excuse not to sign the deal. She reportedly said she would not sign it as her husband was away on a business trip. What happened after still remains a mystery today. Gary said that the deal was finally reached but it was IBM that did not honour the agreement.
Whatever the case may be, one thing we are sure of is that IBM moved on from the then quite popular CP/M to a deal with Microsoft, which agreed to purchased a license to a CP/M clone that we now know as 86-DOS. They quickly adopted 86-DOS for the IBM’s new PCs, of course after major changes were made, and branded it as the MS-DOS, even though it was called PC DOS by the IBM.
Being based on the CP/M clone, MS-DOS clearly borrowed the disk drive lettering schema from CP/M, which had borrowed it from the earlier IBM systems. By copying many of elements of the CP/M system, it can easily port popular software packages that were running on CP/M to to the MS-DOS which was based on the new IBM PC.
Now, this brings us back to the drive lettering schema. You may have notice that most of the earlier computers usually don’t come with internal mass-storage devices mainly due to its high cost (even though HDDs had been there since 1950s). It was an era of the “floppy” disk reader and the 5 1/4″ floppy disks used to be inserted into labelled “A” in MS-DOS and in other operating systems. Some PCs came with two floppy disk drives necessitating the need for an extra drive “B”. When the 3.5″ floppy disk was commonly added and used, “A” and “B” drives were specifically meant for floppy drives.
Finally, when hard disk drives became standard features in most Personal Computers in the late 1980s, a need was felt for another drive in the gadget. Since, the first two letters in the alphabet - A and B - were already used for floppy drives, the hard disk which was considered a third drive logically got “C”. Today, most systems have done away with the floppy disk but the schema of drive designation remains, with “A” and “B” drives by default reserved for floppy drives. Even though these drives are non-permanent and can be easily changed or altered if you have administrative rights, the protocol is followed to today and ‘C’ continues to be the default drive in most PCs.