We've introduced you to M.2 drives and 2.5-inch drives above, but let's get into them in a bit more detail. The 2.5-inch Serial ATA SSD was one of the earliest consumer-facing implementations of SSD technology and remains popular, especially for upgrading older PCs. While the drive electronics are much smaller than 2.5 inches, its enclosure will measure a bit wider (actually 2.75 inches wide, despite the name), so it will fit into the same mounting brackets in your desktop or laptop used by 2.5-inch hard drives. That makes them your most likely choice for upgrading a platter-based boot drive in an older laptop. And almost any desktop PC nowadays will have 2.5-inch bays, or let you install a 2.5-inch drive in a 3.5-inch hard drive bay. If you're upgrading an older laptop that has a 2.5-inch bay (most recent ones won't), you'll also want to account for the thickness of a 2.5-inch SSD. Almost all SSDs nowadays conform to a 7mm thickness, but older laptops with SATA hard drives may have drive bays with as much as 9.5mm clearance. Some makers of 2.5-inch bundle a space-filling frame with their drives to keep a thinner 7mm drive from rattling around in a roomier bay. That's less common today than in years past, though. You may have to get one of those aftermarket if you're upgrading an older laptop and it needs the filler. M.2 slots are now near-ubiquitous in new desktop motherboards and common in late-model laptops (where the drives aren't just soldered down and not upgradable). M.2 solid-state drives are the 2.5-inch drive distilled to its essence, extremely minimal in their design and implementation. But they're also the most complicated to understand before you buy. First, consider the bus type. M.2 drives come in SATA bus and PCI Express bus flavors, and the drive requires a compatible slot to work. Some M.2 slots support both buses on a single slot, but drives support just one or the other, so make sure the SSD you buy matches the bus type available on the slot in question. The vast majority of recent M.2 SSDs are PCI Express NVMe drives, so if you have an older computer with a SATA-only M.2 slot, finding a compatible SSD for it will require a bit more hunting. M.2 drives also come in different lengths. Physically, the most common of five M.2 SSD sizes is what's known as Type-2280, shorthand for 22 millimeters wide and 80mm long. (All SSDs you'll see for consumer PC upgrades are 22mm wide; lengths range from 30mm to 110mm.) Most are merely circuit boards with flash memory and controller chips on them, but some M.2 drives (especially those of the PCI Express 4.0 and 5.0 variety) now ship with relatively large heatsinks mounted on top to keep them cool, or in the box as accessories. If an M.2 drive you're looking at has one of these special, big heatsinks on it, make sure your desktop's motherboard has the clearance above and around it to accommodate its bulk. Some desktop motherboards situate an M.2 slot right alongside the ideal expansion slot you'd use for your graphics card, for example, and the hardware can collide. Laptop designs typically can't stomach a special, tall heat sink at all.