External drives fail in two very different ways, and the difference matters a lot for your data. If the drive is electrical (board, cable, power), the data is usually safe and the fix is cheap. If it's mechanical (the disk spinning, clicking, or grinding), every extra spin-up risks destroying the data. So the first job is to listen and decide which one you have before doing anything.
Listen to it
When you plug the drive in, listen and feel. A healthy drive makes a soft, steady whir. Clicking, grinding, or repeated spin-up-then-stop is a mechanical fault. If you hear that, unplug it immediately and don't try to read it again — take it to a data-recovery service. Continuing to power a mechanically failing drive is the single most common way people make an unrecoverable drive unrecoverable.
Give it real power
If the drive has its own power adapter, use it — every time. A 3.5" drive that's only getting USB power will behave erratically. If it's a bus-powered 2.5" drive, plug it into a port on the back of the PC (or a powered hub), not a front port, and try a different cable. A weak or frayed cable is the most common 'detected sometimes' cause.
Check it's actually connected
Right-click Start → Disk Management. If the drive appears there (even as 'Unallocated' or without a letter), the PC sees it and the problem is a missing drive letter or a filesystem issue — right-click the drive → Change drive letter and path → Add. If it doesn't appear at all, the fault is power, cable, or the drive's board.
Try it on another computer
This is the cleanest test. If it's detected on a different PC, the original port or system is at fault. If it fails everywhere, the drive itself (board or mechanics) is the problem.
When to stop
If the drive makes no sound at all (no whir), it's not getting power or its board is dead. If it makes a whir but isn't detected, suspect the cable or port first. And if it clicks or grinds — that's your cue to stop and seek professional recovery, especially if the data matters.
The one rule that saves the most data: if it clicks or grinds, unplug it and stop. Mechanical failure gets worse with every attempt, and a recovery lab has tools you don't.