Install Win8 on external HDD from HP recovery DVD

So I'm using linux on my hp laptop where was previously windows 8. I have made backup of win8 with HP recovery manager on a dvd and completely formated hdd. Now I want to install windows 8 but only on external hdd. I have tried using official windows 8 iso but it says that my key cannot be used to install retail version of win8.
Does somebody know how to do this?

8.0 ISO or 8.1 ISO? You can't use a Windows 8.0 key to install Windows 8.1. You can do this though, http://www.eightforums.com/installa...retail-windows-8-1-windows-8-product-key.html. If your trying to install 8.0 then your original install must have been the Single Language version. That's a special ISO. Regular Retail DVD, as far as I know, will read and use the OEM embedded product code automatically and won't prompt you to enter a code. My 8.0 TechNet ISO's work that way on my laptop that came with Windows 8.0 Core.

Ops, forgot to mention, no you can't install to an external hard drive. Other than Windows To Go, which is only available in the Enterprise edition, Windows won't let you install to removable media.