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Western Digital has been substituting inferior SMR (Shingled Magnetic Recording) disks in place of earlier CMR (Conventional Magnetic Recording) disks. SMR is a technology that allows them to eke out higher storage densities, netting more TB capacity on the same number of platters—or fewer platters, for the same amount of TB (AKA: cheaper to produce). The problem is that in addition to higher capacities, SMR is associated with much lower random I/O performance than CMR disks offer. SMR disks also have problems with being dropped from arrays, if they work at all. If you look at the model number, EFRX model is a faster CMR drive, while the EFAX models are SMR drives with extremely bad sequential write speeds. The SMR drives also require a larger cache to try and help with the slow write speeds.