Recent stats say that by far most people running 10 do not have the Anniversary Update build installed -- the don't [can't] say whether that's by user or Microsoft choice. If there comes a time when you're stuck without a choice, & the update breaks your device, maybe something here will help?...
I've got 3 copies of 10 on this PC, plus a few VMs. Why? They each have a few purposes & I've got the disk space. But that's neither here nor there... I had problems upgrading 10 with only one copy, and weighing the expected cost in time & effort to figure out what was wrong [which might after all be incurable] against the cost of starting over with a fresh install, I chose to install the new version of 10 fresh. If it had been the copy of 10 where I have much more software installed, I'd have tried harder. So what can you do when the upgrade doesn't work, short of a fresh install?...
I'll start with the usual recommendation to do a full disk image backup -- none of this is non-destructive including the upgrade, whether it works or not. Windows Central has an article listing several possibilities, things you can try, but please take them with a grain of salt, e.g. error messages & numbers are hard-coded, & stuff that the developers didn't think of can trigger them, so what's reported can be flat out wrong. IOW if one thing doesn't work, try something else. windowscentral[.]com/windows10-anniversary-update-common-problems-how-fix
Something else to bear in mind, the writers at sites like Windows Central are just that, writers -- even Windows experts can be wrong, & these are not experts, so please take what they write as more of an educated opinion rather than fact.
Another article at Windows Central talks about Microsoft working on a problem with upgraded Windows 10 causing devices to freeze. They tallk about one option, or rolling back to the previous build, and say there's on on-going discussion in a linked forum thread. windowscentral[.]com/some-windows-10-anniversary-update-pcs-experiencing-freezing-issues
When a Windows 10 upgrade [or new version fresh install] fails, the best thing to do may be to restore that backup & simply wait -- there's a decent chance Microsoft will fix it. When you run Windows 10 setup in Windows it asks if you want to check for updates -- if you check the yes box it'll update the new version of Windows before it upgrades your existing copy; upgrading via Windows Update should do the same thing. But waiting may not be an option...
If the upgrade doesn't work you can enroll that copy of 10 in the Insider program, but while that will get you newer versions or builds of Windows 10 that are hopefully fixed, there are big downsides... It may take more than one new build before a fix shows up, & there's always the chance that a fix never happen. And even if you put that copy of 10 in the so-called Slow Ring, where you get fewer but better quality pre-release builds, you'll still be running beta versions, plus you'll have to install several -- opting out means going back to the last general release [that didn't work]. There is however another, unsupported option...
When you upgrade Windows 10 builds or versions there's a file called install.esd left behind, & you can use that file to create a Windows setup disc ISO. [C:\$WINDOWS.~BT\Sources\] That ISO can be used to install that build of 10 fresh or upgrade an existing copy. So if you have an Insider copy of 10 on a device or in a VM that you successfully upgraded to a new build, you can upgrade a copy of 10 that's not in the Insider program. Note that both versions of 10 have to be the same, as in Pro for Pro, 32 bit for 32 bit. Note also that you may get nags that something's wrong with the parts of 10 that get you new Insider preview builds etc. -- as I said this is completely unsupported, but I've done it a few times & it's worked.
Drivers can be a problem with 10... There are a lot of devices & components that are made & sold for a limited time by companies with no intent of having a long term customer base or relationship, e.g. company A wants to sell a new laptop model, & contracts with company Z in Asia to make a production run of some component it will use. Company Z may never make that part again. And if that part uses a driver, the one that's initially delivered to company A may well be it -- there may never be a later version. Windows 10 may, among other things, change the way it uses that part, or the way it works with the driver, or the certification requirements for drivers. During setup 10 may use the wrong driver or not install a driver at all, when the old driver, or even a driver for win7 or 8 will work OK. Sometimes that can prevent Windows 10 from starting after a new install or upgrade.
One way that might get around that is fixing the driver issue in Safe Mode, uninstalling the device or updateing the driver manually in Device Mgr. Another way to try that might also help with diagnosis is a somewhat lengthy work-a-round... Perform a disk image backup of the device as it is when 10 will not start [or freezes almost immediately] using the backup software's bootable media, e.g. USB stick. On another Windows device create a new VHD, mount it, & restore that backup to it. Then dismount the VHD & use it as the basis for a new VM, e.g. in VirtualBox. It should start, & use the drivers that match the VMs virtual hardware -- optionally you can check Device Mgr. & uninstall any old drivers that aren't being used. Then close the VM, mount the VHD, perform a disk image backup, & restore that to the original device where you're having problems.
Hopefully Windows 10 should start on the original device now, but there are going to be devices in Device Mgr. that are either not working at all, or are using the wrong driver, but with 10 running now you can individually update them. A variation on this method is to restore a disk image backup of the problem device before the upgrade was attempted to a VHD, then performing the upgrade with that copy of 10 running as a VM. Either way *may* be the only way short of building a custom wim image to get some drivers installed. It's a bit of work, & it may or may not work, but if the only other option is a bricked device, it may be worth it.
