"Diagnostic Boot Selected" problem on power-up boot

its the problem with the keyboard. make sure Function key is not pressed while turning on the computer. normally it happens only then. if the problem presists try reseat the keybaord. to see how to reseat the keyboard.
NOTICE: Disconnect the computer and any attached devices from electrical outlets, and remove any installed batteries.
NOTICE: To avoid ESD, ground yourself by using a wrist grounding strap or by touching an unpainted metal surface on the computer.

1. Remove the hard drive.
   
   
2. Turn the computer over, and remove the five M2.5 x 12-mm screws labeled "circle K."
3. Turn the computer over and open the display.

NOTICE: The keycaps on the keyboard are fragile, easily dislodged, and time- consuming to replace. Be careful when removing and handling the keyboard.

1. Remove the center control cover:
   
   
2. Insert a small, flat-blade screwdriver or plastic scribe into the notch at the right end of the center control cover and pry up the cover.
   
   
3. Lift the center control cover up and away from the bottom case.
4. To release the keyboard from the palm rest, use a small, flat-blade screwdriver or plastic scribe to pry up the right edge of the keyboard near the blank key.
5. Lift the keyboard straight up about an inch and then rotate it forward toward the front of the computer, until the key face is resting on the palm rest.

NOTICE: Do not pull on the keyboard flex and track stick cables.

Pull up on the keyboard connector to disconnect it from the keyboard interface connector on the system board.
Remove the keyboard from the bottom case

See the images below for ready refference




To repalce the HDD see the image below


I am sure this will fix the computer. Because in this you are reseating the HDD as well. all the best.