OK some googling later .

UEFI seems to be involved .ASUSTeK model: P9X79 is one of the spanking new mobos that supports this

Win 7 boots from a separate EFI partition .

While Mint does not use EFI it uses normal bios boot mode

Apparently EFI was something pioneered on never intel MAC's but it is comming to PC too, I think we can expect a lot of bios upgrades before this functions properly .
The intention is to actually have a better boot manager ......

There you have I just bought the PC and let the guys in the shop put it together so a few surprises are probably expected. I specified performance and they helped to pick suitable hardware

Well next year I assume all Linux distros are up to speed with EFI

There is the rEFInd boot manager of course but it is currently not a n00b endeavour to install.