When using RAID as an OS Volume, it's important to ensure that all volumes are used for Boot. Certain RAID configurations will require that any drive within the RAID be available as a Boot volume. While UNIX and Windows installers may correctly setup the boot partition during installation, the BIOS will need to updated to ensure that the drives are used as boot drives. 

One area that is often overlooked are "INT 13 Devices" 

INT 13 Devices is used to set devices under a controller as Boot Devices. 

If a controller has 3 Disks set to a RAID 1E (3 Disks configured for RAID 1) then you will need all 3 INT 13 Devices set within the BIOS. This means that 3 different Devices can be allocated for Boot Volumes. This number should always reflect the number of Boot Volumes within your RAID/OS volume. 

The ability to set the Maximum INT 13 Devices for this Adapter is presented within certain HBA and RAID controllers. When booting, there’s an opportunity to hit CTRL-C to enter the HBA config. Under Advanced Adapter Properties > Advanced Device Properties-> Change the entry "Maximum INT 13 Devices for this Adapter” from 1 (the default) to 3 (the total number of disks).

Even though MBR, GRUB or other boot volumes may be available to the system, this setting must reflect the total number of boot volumes or the system will not boot correctly.