The Unestech ST3540U2, a Tray-less SATA 6Gbps HDD Mobile Rack for 3.5" SATA hard drives. 

It is designed to maximize space use by allowing up to 4 x 3.5" SATA hard drives, while only consuming 3 x 5.25" bays. Connects 4 x 3.5” SAS / SATA drive through 1 x MiniSAS HD (SFF-8643) connector,transfer speed up to 12Gb/s for SAS drives & 6Gb/s for SATA drives.

Tray-less design, no screws or tools are necessary to insert or eject the drives, making hot swapping a completely hassle free experience.