I've just heared about the horrific stuff that should be patched in this update so had been a little sad seeing the result of the update:

Cumulative Update for Windows 10 Version 1607 for x64-based Systems (KB4015217) Failed to install on 2017.04.12.

Well thats anything but helpful so checked %WINDIR%\Logs\CBS\CBS.log for something more specific.

Almost at the end of the file I've seen the following:

Ok. So It's once again a problem of having a small SSD and a larger HDD and that I've moved a few folders to the hdd after the install of visual studio took most of the free space I've had. Let's continue..

Just scrolling backwards in the file I've found multiple error messages which all seemed to indicate that there is something wrong that caused multiple problems during the update process.

Finally I've reached this message:

(F) Error: ResolvePendingTransactions called after poqexec failure (call 1) Status = STATUS_NOT_SAME_DEVICE, Operation = HardLinkFile, DiagString = [l:254]'\SystemRoot\WinSxS\amd64_microsoft.virtualiz..ent.rdpclientaxhost_31bf3856ad364e35_10.0.14393.1066_none_1d0de1e8d01ff5a1\Microsoft.Virtualization.Client.RdpClientAxHost.dll, \??\C:\Program Files\Hyper-V\Microsoft.Virtualization.Client.RdpClientAxHost.dll'

The important part is

Operation = HardLinkFile

and the path:

C:\Program Files\Hyper-V which indeed was a juntion to the hdd. Renamed the junction, copied the files back, initiated the upgrade once again..

..and finally it works..

==Enjoy =) ==