Stefan Mileschin

Apple's Retina MacBook Pro and all but the earliest MacBook Air models have relied solely on SSDs for internal storage, as Apple slimmed down the designs to the point that even a 1.8" hard drive was too bulky. Rather than adopt the mSATA or later M.2 form factor, Apple's SSDs have used custom form factors and pinouts. This has contributed to keeping the market for third-party upgrades very small. Only a few companies have produced SSDs in Apple-specific form factors, most notably Other World Computing (OWC) and Transcend. Transcend has generally used Silicon Motion controllers while OWC has used SandForce controllers, but until now their offerings have been limited to SATA-based SSDs.

http://www.anandtech.com/show/10128/...-air-pcie-ssds