Asus TS300-PA4 - Manual (Page 180)

6-52 Chapter 6: RAID configuration
2. Select the RAID set you want to make bootable, then press <Enter>.
4. When prompted, select Mark
bootable, then press <Enter>.
3. Press <B> when the RAID set information displays on screen.
The letter “B” appears before a bootable RAID set for easy identification.