Why is my external hard drive not showing up?

Determine first whether the drive is being detected at all, because that single fact separates two very different problems. The disk management utility on your system lists physical disks regardless of whether they are mounted or readable, so a drive appearing there but not in your file browser is connected and simply not accessible.

When the drive appears but is not accessible, the usual reasons are a missing drive letter or mount point, a partition marked inactive, or a filesystem the operating system cannot read. That last one is common when moving between operating systems, since each has a native format the others handle poorly or not at all. The drive is fine; the machine cannot interpret it.

When the drive does not appear at all, work through the physical path. Try another cable, since data-capable and charge-only cables look identical. Try a port directly on the machine rather than a hub. Listen for the drive spinning up on mechanical models, which tells you whether it is receiving power.

Power is the most frequent cause with portable drives. They draw more current than many ports supply comfortably, and an underpowered drive will spin up, be detected briefly, and vanish, sometimes repeatedly. A powered hub or a dedicated high-power port resolves it.

If the drive is detected intermittently and makes clicking or repetitive noises, stop testing and prioritise recovery. Those symptoms indicate mechanical failure, and continued attempts to access it reduce the chance of recovering anything. Imaging the drive to another disk before further troubleshooting is the correct first step whenever the data matters.