Installing Windows 8.1 I get "You cannot install Windows on a USB flash drive from Setup"

I'm trying to install Windows 8.1 RTM (from MSDN) ontop of a Windows 8 installation.

When trying to run the upgrade from Windows 8 running Setup.exe either from ISO or from a USB stick I get:

"You cannot install Windows on a USB flash drive from Setup"

Windows 8 is installed on C: and there are no connected USB drives when installing from ISO, or the USB key when trying to install from that.

I've also tried installing off the bootable USB key by booting from there, which gets me to the point of selecting a drive to install on at which point it tells me that there's no valid partition.

Other info:
I deleted the Windows recovery partition some time ago and when running the full setup from the USB key the 8.1 installer created a new empty one (even before it asked permission to install anything). So now I have an extra partition that's showing up as an extra drive letter and I can't remove it even using DISKPART.

Installed 8.1 on another machine today and that worked without issues, so it seems clear that this is related to the machine config somehow.

Any ideas what the installer is getting hung up on here?

Answer

Check this thread: http://answers.microsoft.com/en-us/windows/forum/windows8_1_pr-windows_install/you-cant-install-windows-on-a-usb-flash-drive-from/be16d060-7b7f-48fa-b56e-4faf7cd28a6d?msgId=5cbe976a-de77-45a0-93c3-82aa860dcc38

Especially part about:

change the value of PortableOperatingSystem in regedit HK_Local_Machine/System/CurrentControlSet/Control/ from 1 to 0

It worked for me.