Zigbee coordinator on a USB 3 port or next to an SSD: expect a weak, unstable network

Symptom

Devices drop off, pairing fails or only works right next to the coordinator, the network map shows poor link quality, even though everything is in the same room.

Why

Zigbee uses 2.4 GHz with little transmit power. USB 3 ports and devices, SSDs, HDMI ports and Wi-Fi routers radiate noise in that band. A coordinator plugged directly into a single-board computer or mini PC sits right in that noise.

Zigbee2MQTT's stability guide puts it bluntly: an adapter placed close to a USB or HDMI port can lose its radio signal entirely.

Fix

1. USB extension cable. Zigbee2MQTT: 50 cm is already enough to reduce interference. Home Assistant recommends a long, shielded one.
2. USB 2 port, not USB 3. If the host only has USB 3, the extension cable (or a USB 2 hub) matters even more.
3. Distance from SSDs, USB 3 devices, HDMI and the Wi-Fi router. Don't put the adapter on top of the router or next to an external SSD.
4. Try the orientation. Zigbee2MQTT notes that the adapter's orientation in space affects the link; rotate it while watching link quality.
5. Then look at the Zigbee channel vs your Wi-Fi channel.

This is the cheapest fix for most "Zigbee is unreliable" reports: check it before buying more hardware.