Those hdd holders are well designed, allowing 2.5 and 3.5 and the 3.5's are mounted with antivibration grommets. It would be possible to mount a 3,5 and a 2.5 on opposite sides of one of these mounts. Of course the trays could not sit in adjacent bays, but you could fit 2+2 in 3 bays. This might be handy if using very long graphics cards. I have an R3.