Why does my USB device not show up when I plug it in?

Work outward from the simplest explanation, because the cheap tests eliminate most causes quickly.

Try a different port first, ideally one directly on the machine rather than through a hub. Ports fail individually, and front-panel ports on desktops are connected by internal cables that come loose. Hubs introduce their own failures, particularly unpowered ones supporting devices that need more current than the hub can supply.

Try a different cable next if the device uses a detachable one. This is the highest-yield test of all for phones and portable drives, because many cables are designed only for charging and carry no data connections at all. A device that charges but does not appear is very likely on a charge-only cable.

If the device appears in the system device list but does not work, the problem is driver rather than connection. A device showing with a warning indicator is being detected but not usable, and reinstalling or updating the driver is the path forward. External drives specifically may appear in disk management without being assigned a drive letter, in which case they are connected and simply not mounted.

Power is a genuine constraint for portable drives and anything with a motor. A drive that spins up and then disconnects repeatedly is usually not receiving enough current, and a powered hub or a dedicated port resolves it.

Finally, formatting matters for storage. A drive formatted for another operating system may be detected without being readable, appearing to fail when it is simply using a filesystem the machine does not understand. Disk management will show the partition exists, which distinguishes this from a hardware fault immediately.