Hi and thanks for your answer.
That's what i tried before i registered with Intel Communities.
I enabled the CSM in BIOS and went to the familiar [Ctrl]-[I] but the selection is between RAID0 (Stripe). Full stop.
Updated the driver using the stock Windows 8.1 that came on the SSD, but sadly that didn't change the BIOS part of Intel RST which remained 18.104.22.1686
What next? Ask ASUS if there's a BIOS Version 207 in the pipeline? Is the ASUS team responsible for not implementing that part of the chipset's features?
Thanks Joe for finding out,
Updating the Intel® Rapid Storage Technology software driver in Windows* will not change the Intel® RAID Option ROM version. Updates for the Intel® RAID Option ROM version are usually provided via BIOS updates by the system manufacturer.
You may try resetting the disks to non-RAID first or try creating the RAID 1 in Windows* with Intel® Rapid Storage Technology as indicated in pages 30-33 of the user guide. Otherwise, contact the system manufacturer to find out what are the supported features.