The IcyDock MB994SP-4S is just such a unit - it packs four 2.5" HDD/SSD drives into the space of a single 5.25" DVD drive and provides unified power and supported SATA hook ups at the rear. On top of its high density convenience, all four SATA drive bays are hot swappable; without popping a side panel off , without unscrewing anything and without connecting any cables.
A pair of 40mm fans built into the rack even ensures a modicum of airflow to keep drive temperatures in check.
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