Have revisited this issue and tried every possible config of single drive 500gb, two drives 500gb each [unconfigured], two drives 500gb each in raid0 and then the same again but with 2tb drives.
None of them will allow x64 to be installed.
Please Intel, tell me I'm doing something wrong here. Live chat have advised that this should work.
The board is a brick to me unless this can work.
Maybe it did work with the original bios and now it has been broken in 0525?
Edit: I should add here that x64 boot on marvell is possible but only if you disable uefi, which means no more than 2tb boot volume on either controller.
So the options are:
Single sata3 boot volume of any capacity
Dual sata3 boot but only to volumes of <2tb
I managed to get Win7 x64 to run on the Intel RAID, but it required a lot of troubleshooting by Intel and they finally supplied me with this driver:
It's worth a try if other people find this and can't get the RAID to work.