Western Digital WD Blue SSHD WD40E31X, 4TB, SATA III, 3.5in, 64MB Cache. WD Blue Solid State Hybrid Drives (SSHD) combine a generous amount of NAND flash SSD technology and massive HDD storage capacity into a single drive. WD engineers produce a hybrid that can increase overall system performance – resulting in quicker system boots, faster application loads, and improved responsiveness when compared to traditional hard drives.