(Credit: Pathdoc/Shutterstock)

In the wake of the unexpectedly messy October update for Windows 10 that ended up stretching well into November, users of Microsoft's operating system may be scratching their heads about how it went so badly. While Windows 10 isn't as widely praised as Windows 7, it rarely experiences the issues that we saw recently, which included deleting entire directories of files.

To address the growing uncertainties about Windows 10 and the integrity of its update system, corporate VP Michael Fortin took to the company's blog this week to explain how Microsoft's system works. Unfortunately, some of the details he revealed may cause more concern than confidence.

SEE: Microsoft Edge beta for Android now syncs with your Windows 10 timeline

Fortin says that each week of a month is internally assigned the letters A through D. So for example, Microsoft refers to Patch Tuesday as the "B" update, because it always occurs in the second week. He adds that these are "the only regular monthly releases that include both new security fixes and previously released security and non-security fixes."

Now, if you are the type of Windows 10 user who manually checks for updates, you've probably seen downloads popping up in the "C" and "D" weeks of the month. Microsoft, or at least Fortin, refers to this user practice as "seeking," and he has some unexpected things to say about the updates that become available during those parts of the month.

He says, "We also release optional updates in the third and fourth weeks of the month, respectively known as 'C' and 'D' releases. These are preview releases, primarily for commercial customers and advanced users 'seeking' updates. 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.

"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."

FOLLOW Download.com on Twitter for all the latest app news.

As you might imagine, there are at least two potential problems here. One, Microsoft is providing updates to regular Windows 10 users that may not have been fully tested before deploying to the public. Until now, this sort of update was generally understood to be limited to the Windows Insider program, the company's official beta testing system where users specifically opt in to test updates themselves and are explicitly made aware of the potential risks.

The Insider version of Windows 10 is officially discouraged for use in production environments, so exposing non-Insider customers to programming code that hasn't gone through a conventional vetting process may be an unwelcome surprise.

The other problem is that a non-Insider user of Windows 10 needs only to manually check for an update during weeks "C" and "D" to be exposed to updates that may not have been fully tested. The implication is that the errors of the October update can be traced back to this practice.

The upshot? If your computing environment requires a verifiably stable version of Windows 10, don't ever tell the operating system to check for an update. This way, the OS will only look for the monthly "B" patch, which the company says has gone through a standard testing process and should be safe for the general public.


  • Microsoft issued a statement on its blog in regards to how it updates Windows 10 and unexpectedly revealed that it's possible for a user to unknowingly download a patch that has not gone through a conventional testing process.
  • Even if you are not an Insider tester, these updates can be obtained simply by manually checking for them in the third and fourth weeks of each month.
  • The implication is that this manual checking process is what led to the widely reported issues with Windows 10's big October update.

Also see

Tom McNamara is a Senior Editor for CNET's Download.com. He mainly covers Windows, mobile and desktop security, games, Google, streaming services, and social media. Tom was also an editor at Maximum PC and IGN, and his work has appeared on CNET, PC Gamer, MSN.com, and Salon.com. He's also unreasonably proud that he's kept the same phone for more than two years.