The drive spins but will not mount: cable, power, letter, format - the three-check ladder

A drive that clicks or whirs at a stranger's laptop and does nothing on yours is rarely a dead drive - it is a drive nobody completed the handshake with. The ladder below is ordered by how fast each rung clears: cable and power first because they cost a minute, Windows' hidden volume decisions second because that is where the quiet half of the problem lives, and the drive itself last because that is where the money lives.

Minute one: the cable is guilty until proven innocent

• Charge-only cables are the most wasted hour in IT. A micro-USB or USB-C cable from a drawer may carry power and no data - the drive spins up beautifully and stays invisible. Swap to the cable the drive came with, or the phone's good data cable, before reading anything else here.
• Power, especially on big metal externals: desktop 3.5-inch drives have their own adaptor - check it is the right one, the wattage printed on the brick, not the voltage alone; bus-powered 2.5-inch drives die silently on weak ports - rear motherboard ports beat front-panel hubs beat unpowered USB-C dongles, and an externally powered hub settles the question outright.
• One machine or two: "shows on the other laptop" is the whole diagnosis - the drive lives, and rung two is your machine's problem.

Rung two: Windows has it and is not telling anyone

Disk Management (right-click Start - Disk Management) is the screen that ends the argument, because a volume that mounted for years can go letterless silently when it returns from another machine's file system. An unallocated drive that once held data is not unallocated - that is a partition table confused by a second cable or a failing sector, so stop and read the next section before clicking anything a dialog suggests. What each state means: healthy volume without a letter - right-click - Change Drive Letter - assign one; dynamic or foreign disk - import it; a raw file system where a partition should be - do not let Windows "format to initialise", that is rung three's mistake on the way to rung four; and an external that vanishes under load - the drive's own sleep or selective suspend, both switchable off in power settings.

Rung three: the drive itself, and the enclosure trick that splits the cost

Before mourning the drive, free the drive: a "dead external" is half enclosure and half disk, and the plastic sandwich is a five-dollar part - pop the disk out (most externals are a 2.5-inch SATA drive behind a USB bridge; some NVMe sticks behind a small board), seat it in a known-good enclosure or cable adapter, or a desktop's SATA bay. If it mounts there, you bought an enclosure and kept the data; if it does not, the disk is the fault, and the behaviour tells you which fault: recognized with slow, stuttering reads and growing SMART pending-sector counts is a drive to copy immediately, one pass, by largest file first - not defrag, not chkdsk, the repair tools finish a dying drive's sentence. And the sound that ends the ladder: a drive that clicks or beeps, mounts for seconds and drops - that is heads or a seized spindle, power it off permanently, and data-recovery benches are the only next stop for anything that matters. The prevention footnote the desk insists on: exFAT is the cross-platform format that gets drive-letters right far more often than NTFS's read-only stubbornness on Macs, and a drive on the shelf, unpowered, in humidity, is the last rung none of these fix - spin the archive quarterly; that is the free warranty.