External hard drives are more fragile than they appear, and plugging one in after a drop can cause permanent data loss. The read/write heads and platters operate just nanometers apart, so even a minor impact can misalign them. When you plug in a damaged drive, the spinning platters can scratch against the heads, destroying data and creating debris that causes further damage.
This is especially risky if the drive was running during the fall. Technicians recover data by replacing damaged parts, but repeated spin-ups after injury make recovery harder. If your drive falls, treat it as damaged until professionally evaluated.
Do not plug it in, tap, shake, or freeze it. If you must test it, listen for abnormal noises like clicking or grinding and unplug immediately if heard. Check SMART data for warnings like reallocated sectors, and back up important files first.
Source: Engadget · Summarized by HeadlinesBriefing