SSD

The 2.5-inch SATA SSD shares its physical footprint (100mm x 70mm) with laptop hard drives, making it a drop-in upgrade. SATA III limits it to a real-world maximum around 550-600 MB/s regardless of how fast the drive's flash memory could otherwise go.

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M.2 is the dominant SSD form factor today, sized by a code like 2280 (22mm wide, 80mm long). B-key supports SATA or PCIe x2; M-key supports full PCIe x4 NVMe speed; B+M-key dual-notch modules are actually capped at PCIe x2, not the full x4 the two notches might suggest.

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mSATA (Mini-SATA) packed a SATA interface into the same physical card size as Mini PCIe, standardized in 2011 -- but despite sharing its connector and outline with Mini PCIe, it's electrically incompatible, carrying SATA signal lines instead of PCIe lines. M.2 has now largely superseded it.

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PCIe Add-in Card (AIC) SSDs, commonly called HHHL (Half-Height, Half-Length), plug directly into a PCIe expansion slot like a graphics card rather than using M.2 or U.2 connectors -- trading compactness for the far greater power delivery and cooling surface area a full expansion card provides.

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U.2 uses the SFF-8639 connector to bring PCIe x4 NVMe speeds to a 2.5-inch, hot-swappable drive body -- the enterprise and datacenter counterpart to consumer M.2, trading M.2's compact size for better cooling, higher power delivery, and the ability to swap drives without powering down.

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