When SSDs were first introduced into the market, manufacturers' made sure that they fit into the slots and bays initially used by the HDDs. As such they had the 3.5 and 2.5 inch form factors. However, as technlogy advances, more form factors emerged. Smaller form factors that directly interfaces with PCIe and NVMe slots emerged. These incuded the mSATA, PCIe and M.2, form factors. 