In our Emulab testbed, the boot path for nodes is to have them PXE boot our custom version of Grub (or the FreeBSD boot loader). That boot loader contacts the Emulab server which tells the node what to do next. Typically it is told to boot from a partition on the first hard drive, an action that is implemented by using the BIOS routines to load a sector from disk and jump to it.
However on our NUC5i5MYBE, when we network boot in this manner, the SATA drive is not visible to the boot loader (i.e., no BIOS drive 0x80). This leads me to believe that the SATA/AHCI option rom is not loaded unless you actually boot from the SATA disk. A bootloader booted from a USB dongle likewise cannot see the SATA disk. We are using the latest BIOS (BIOS MYBDWi5v.86A.0024.2015.0504.1120 05/04/2015).
There are obviously plenty of ways around this, e.g., net booting a version of Grub with its own builtin AHCI driver, but I am just wondering if this is a bug or a feature or a misconfiguration on my part!