So a while back, I heard that MSDOS was originally named QDOS (Quick and Dirty Operating System), and that it was later changed is MSDOS (Microsoft Disk Operating System). This was probably a marketing strategy to fix a rather unappealing name. My question is when did this change take place, and why did it take place specifically then?
MS-DOS was indeed originally known as QDOS. The change in name occurred as a result of a change in ownership — from Seattle Computer Products to Microsoft.
Tim Paterson, QDOS’ author, has documented this:
The first versions of the operating system, called QDOS 0.10, were shipped in August 1980. QDOS stood for Quick and Dirty Operating System because it was thrown together in such a hurry (two man-months), but it worked surprisingly well. [...]
In the last few days of 1980, a new version of the DOS was released, now known as 86-DOS version 0.3. [...]
In July 1981, Microsoft bought all rights to the DOS from Seattle Computer, and the name MS-DOS was adopted. Shortly afterward, IBM announced the Personal Computer, using as its operating system what was essentially Seattle Computer's 86-DOS 1.14. Microsoft has been continuously improving the DOS, providing version 1.24 to IBM (as IBM's version 1.1) with MS-DOS version 1.25 as the general release to all MS-DOS customers in March 1982.
IBM provided DOS with their PC as “PC DOS”.
An interesting artifact came to light earlier this year: a copy of Seattle DOS 3.1! As part of the sale of 86-DOS to Microsoft, SCP were granted a royalty-free license to DOS, for use with their computer hardware, and evidently continued to produce OEM versions of MS-DOS at least until version 3.1. (Microsoft bought the license back at the end of 1986.)