USB is one of the great success stories of computing — billions of devices, all sharing one connector standard. The downside is that there are a lot of moving parts behind the scenes. When you plug in a USB device, the operating system has to: detect that something was inserted, ask the device to identify itself, find the right driver to talk to it, load that driver, and finally let your applications use it. A failure at any of those steps shows up as 'not recognised'.
The single most common cause is the cable. USB cables — especially cheap ones — wear out, develop intermittent breaks, or were never built to carry data in the first place (some are charge-only). Always rule out the cable before suspecting anything more complicated.
The second cause is the USB controller driver itself, which sits one layer below the device-specific driver. If the controller is in a weird state, no USB device on that port will work properly. A clean reinstall of the controller driver fixes this kind of system-wide misbehaviour.
Work through these in order. Most readers solve the problem by step three — and almost everyone by step five.
Before any software fixes, do the physical-layer check. Different port, different cable. This solves the problem about a third of the time on its own.
Open Device Manager and look for any device showing a yellow warning triangle. That is the OS telling you exactly which driver it cannot load.
If the issue affects multiple devices, the USB controller driver is the suspect. Uninstalling it forces a fresh load on next boot.
On laptops, the OS can power down USB ports to save battery — and sometimes that goes wrong. Disable selective suspend as a test.
Some USB devices — devices, audio interfaces, complex devices — need their own driver from the maker, not just the generic OS one.
Tap any cause to read the friendly explanation behind it.
Avoid third-party "driver updater" tools that promise instant fixes. They often install bundles you do not need and occasionally cause the very problems they claim to solve. Stick to the official manufacturer support page for your device.
If you have worked through every step and the problem persists, the issue may be hardware, not software. Try the device on a second computer if you can. If it still fails, that is a strong sign the device itself is at fault.
Almost every problem on this page has a fix that takes less time than making a cup of tea. Take a breath, work through the steps in order, and you will get there.
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