Il Mio Problema
The existing 500GB internal drive on my late 2009 21.5" iMac was developing an increasing number of bad sectors, and was probably close to failing.
La Mia Riparazione
It was quite easy, and took less than an hour, with the proper tools.
Il Mio Consiglio
Use a tweezers or small pliers to help extract and insert the Torx screws that attach the display to the case. Otherwise, you risk dropping the screws into the case, which means you'll have to remove all the screws and lift the display in order to retrieve them.