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The white plastic clips that are so hardly visible on a white background are actually brackets for the HDDs and allow them to slide into the drive bays.
The plastic retention clips themselves are a little flimsy .. very bendy plastic really. I like to see something more sturdy resting in rubber mounts.
Above an example of the WD VelociRaptor installed. Bendy brackets or not, I do have to admit that sound dampening works well even with a VelociRaptor installed there's literally hardly any resonating noises etc. Inserting and removing the HDDs is a very simple and smooth process.