A blog post by Michael Fortin, corporate vice-president of Windows, made it clear that apart from the monthly security fixes, which are released on the second Tuesday and known as the B release, the company also releases what are known as C and D releases in the third and fourth weeks of each month.
In October, Microsoft had to halt the rollout of its October update when users reported problems, including the deletion of data residing in some directories.
Users complained at the time that they had lost files from the C:/Users/[username]/Documents/ folder. One user complained that he had lost 220 gigabytes of files which he had accumulated over 23 years.
"These updates have only non-security fixes. The intent of these releases is to provide visibility into, and enable testing of, the non-security fixes that will be included in the next Update Tuesday release (we make these optional to avoid users being rebooted more than once per month).
"Advanced users can access the 'C' and 'D' releases by navigating to Settings > Update & Security > Windows Update and clicking the 'Check for updates' box. The 'D' release has proven popular for those 'seeking' to validate the non-security content of the next “B” release."
There is a category of Windows 10 users known as Insiders who have volunteered to receive updates before they are sent to others. But this post indicates that those who hit the update button during the third and fourth weeks are being used as some kind of guinea pigs for testing beta updates.
Fortin also outlined a number of steps Microsoft takes to ensure that updates are carried out without creating any problems for users. The October experience indicates that this is, at times, not sufficient to prevent problems.