How to open Plextor px-740uf case - help needed

Until now I’ve never found a piece of hardware that I can’t open, but the Plextor PX-740uf DVD burner has me stymied. I found the four removable corner “bumpers” but the exact method to open the case is still a mystery to me. Can someone please provide the answer? Is it related to the slots behind the “bumpers”?
I’m hoping to temporarily use the drive alone, w/o the case and USB/firewire interface. Why, you may ask? I can’t make my Aopen Pentium-M mb boot from the external drive, despite CMOS settings to do so.