When it comes time to install a hard drive, there isn't much to take note of. You take out four screws to allow the removal of the hard drive frame, then install the hard drive. Connect the ribbon cable and the power connection inside the unit and then replace the drive frame into the enclosure. Put the four screws back in and you've finished up the main part of the job at hand. Snap the top cover back into place and you're finished.
Like I said, not too much to it.
Above you'll see the removable frame I was talking about. The screws in the side (included with the unit) are set to allow industry standard drives to be easily mounted. As a side note, make sure the hard drive is set to be a Master drive.
This is what you get after everything except the top cover is in place. Overall a very neat and orderly setup.