I booted my backup PC with Windows 10 Preview on it after seeing the availability of build 10041 announced. The update was there along with a lot of other invalid updates. I boot up WIN10 at at least every other day to see what's new. So I was surprised see so many of these other updates. Many of them were for Intel HD 4600 graphics and ASUS Bluetooth hardware that would be imbedded on the MB. But my Win10 PC is AMD based and has AMD Radeon 3300 on-board graphics and no on-board Bluetooth. The strange thing is that those updates that cause the error are appropriate for my other main PC which is ASUS Z97-PRO and Intel i-469K based with Intel on-board HD4600 graphics... and ASUS on-board Bluetooth. It's spooky but it is as if windows update is confusing its search with my other Windows 7 PC. In WIN10 9926 Windows Update you can't decline an update that the system finds for you.

The system suddenly does a restart and then it can't find the C: drive. I have to power off the power supply for 10 sec and restart to get back to being able to see the SATA1 drive. I had to work through three cycles of those invalid updates and on the last I got the final 21a error code... after that on the next Win10 boot the build 10041 started to install normally.