Clean install confirmed. Upgrade first, activate via online account, then clean install (I did it offline. I always do it offline) skipping all product key entries. 2 strange glitches:
- DVD burn one froze up and wouldn't eject, on restart the bootmgr for Win 10 went missing. Had to restart again to eject and the bootmgr magically re-appeared.
- Clean install restart one, blinking cursor, manual restart and frantic DVD ejection later everything proceeded smoothly from then on. Thought there was another lockup but just another restart. More restarts than 8.1. ~25min or less on this 1TB EVO this is around 1.5yrs old with 4TB written.
- Offline clean install, installed all drivers from mobo site (H81M-S2H), Win 10 reinstalled the HD 4400 drivers. Rest are apparently newer. Or something. And yes you will install them. ALL OF THEM.
Rough but usable so far. Win 8 key did not work. Don't bother.