We're going to:

Swap out a the existing hard drive for a 500 GB drive.

Then remove the optical drive and install another new 500 GB drive in its place.

Carefully insert a putty knife into the crevice between the top cover and bottom housing. Start on the left side first.

Gently enlarge the existing crevice by twisting the putty knife downward and away from the mini.

Repeat the prying motion until a portion of the bottom housing has been nudged upward.