On both of my Windows 10 VMs, I cannot upgrade from build 16199 to build 16215. On the last reboot, it says "boot disk not found". I pdf course tried applying ALL of the usual things, to no avail. Microsoft says they may have fixed this in 16226, but their update system will not let you skip 16215. Does anyone have a solution?
It seems you are trying to upgrade to an Insider Build from an earlier build. Parallels Desktop only supports stable Windows releases. If you still wish to use Insider Build, mount Windows 10 Insider build in the VM and upgrade. If you are facing issues, try uninstalling Parallels Tools and then upgrade. You can take a snapshot before upgrading, so you can easily revert.
you can skip the 16215 update by stopping the BITS and Windows Update services, and then deleting the downloads folder content in c:\Windows\softwareDistribution. Build 16226 fixed one VM, but not the other one.
