Love this build so far. Smooth and fast!
Device activation improvements: We have received a lot of feedback from Insiders on making it easier to activate Windows 10 on devices that take advantage of the free upgrade offer to genuine Windows by using existing Windows 7, Windows 8 or Windows 8.1 product keys. If you install this build of the Windows 10 Insider Preview on a PC and it doesn’t automatically activate, you can enter the product key from Windows 7, Windows 8 or Windows 8.1 used to activate the prior Windows version on the same device to activate Windows 10 by going to Settings > Update & security > Activation and selecting Change Product Key. If you do a clean install of Windows 10 by booting off the media, you can also enter the product key from prior Windows versions on qualifying devices during setup. For more information on these activation improvements including requirements – see this post in the Insider Hub.
"... of the free upgrade offer to genuine Windows by using existing Windows 7, Windows 8 or Windows 8.1 product keys."
Does that means you need to install to a machine with Windows 7, Windows 8 or Windows 8.1 product key first, but not need to go through upgrade procedure?
Firstly, I suspect that MS servers are being hammered for the download right now. Secondly, once you delete the folder and stop/restart wuauserv+bits, (or even toggle Fast Ring/Slow/Fast), I believe you get put at the back of the line. It used to take 24-48 hours for the upgrade to reappear in Windows Update with prior builds.
If you show hidden and system files, look and see if you have the C:\$Windows~BT\Sources folder or C:\$Windows~WS\Sources folder still residing on the machine. If so, there will be an install.ESD or installx64.ESD file. You can use the ESD to ISO tutorial to make an ISO and install the upgrade that way, mounting the ISO by right-clicking, and running setup.exe from within the current W10 OS.
The test pc I will use has vista and 7 dual boot. Just going to try installing on the 7 partition and see if it will accept the key.