A gentle tug on the drive release lever retracts spring-loaded pins from either side of the three drive bays, allowing us to easily free the filler block... full of turtles.*
The same trick releases the DVD+/-RW drive. Ordinary it may be, but useful if you want to re-watch old favorites unavailable on Netflix streaming.
* No turtles were actually provided by Hewlett Packard. iFixit makes no express or implied warranty to the effect that the Hewlett Packard Z820 workstation PC includes turtles, but holds that it would be totally rad.