To get the ASUS X55U to boot first from the DVD drive do the following:

  1. Press “ESC” or “F2” to enter the BIOS setup
  2. Under the “Security” tab, choose "Secure Boot Menu," disable “Secure Boot Control”
  3. Under the “Boot” tab, enable “Launch CSM”
  4. Save the changes and exit
  5. Press “ESC” or “F2” to enter the BIOS setup again
  6. Under the “Boot” tab, the DVD drive and available flash drives will appear. Set the DVD drive as the first boot option. (You might need to have a bootable disk in the drive in order for it to appear as a choice)
  7. Save the changes and exit

Now the X55U will first attempt to boot from the DVD drive. Moreover, if the boot menu is accessed, the DVD drive, hard drive, and USB flash drives (if detected), will appear as boot options.