On the powermac g4 (I have the 1.25 Ghz single G4 model, though I haven't booted it up in years), the mirrored drive covers are springloaded flaps. When you eject a CD, the tray pushes the mirrored flap out of the way.
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The way the Zip drive ejects is not as smooth as an optical drive and would make the door open just a little which may give a feeling of unfinished design. I think every Mac with a Zip drive doesn't have the door for it.