Just checked the BIOS settings (bearing in mind this is a new board to me, I'm used to the old style blue text version with a few options - this ASUS Maximus VIII has about a million and a half settings!) and it's set to AHCI and SATA is enabled, however -- I did have a good read through the manual last night. There are 8 SATA ports, 2 Intel ones, 2 ASMedia ones, and then the 4 shared with the SATA express ports. I think the boot devices may need to sit on the ASMedia ones, when I set it up I just plugged them all in - I suspect that's one reason why I've had issues.

I managed to get Windows 10 finally installed, but now I'm finding it takes around 2 minutes to load, Windows 7 previously took around 7 - 8 seconds as it's on a SSD drive. Had a quick search but again, I'm not sure if this is all related to the same issue.