Update :
I first tried installing 8.1 (got activated) tried, then 10 : no way, 10 does'nt get activated as it asks for a key (no key for Surface).
[EDIT] : secure boot must be disabled !
Then, bare metal restore of 8.1 (reset to factory using a USB stick). Got activated. Installed 10 update (ISO, setup.exe - not boot time). Win 10 got activated. I shrinked partition in order to make room for 8.1.
At this point, one system : activated OEM Win10.
Using Win8.1 ISO, I installed 8.1 on the remaining disk space. It did'nt ask for a serial. In Windows Update, I disabled firmaware update. Why a firmware update ??? Windows 8.1 was also automatically activated.
At this time, I am dual booting an activated 8.1 and an activated 10 on the same Surface Pro 3.
It rocks ! But if something happens that ask me to make a choice, it will be 8.1. Not 10.
Because of this, for example :
https://www.youtube.com/watch?v=x3pCPnGPUtY
post edited by schmorblatz - 2015/08/04 06:38:20