A conversion enclosure allows a first group of hard disk drives of a first size to be received by a hard disk drive enclosure having a second group of hard disk drives of a second size. The conversion enclosure includes a chassis having first and second side panels, each of the side panels having a front edge and a rear edge, a plurality of pairs of chassis rails having a first rail of each of the pairs of chassis rails connected to the first side panel, and the second rails connected to the second side panel. Pairs of slots are included therein, each slot bounded on one side by a first plane passing through a first pair of chassis rails, and on the other side, by a second plane passing through the second pair of chassis rails adjacent to the first pair. The conversion enclosure includes a pair of engaging panels, each having a vented area respectively connected to the rear edge of the first side panel and to the rear edge of the second side panel. The engaging panels respectively engage the hard disk drive enclosure in response to the conversion enclosure being inserted and located into the hard disk drive enclosure.