External hard drives are more fragile than their enclosures suggest. The read/write heads operate just nanometers from the spinning platters, so a drop can misalign them even if the drive looks fine. The data may still be intact after impact, but plugging the drive in can turn a recoverable problem into permanent data loss.
When a damaged drive spins up, misaligned heads can scratch the magnetic surface, and debris from the first scratch can cause further damage. Each additional spin-up makes recovery harder and more expensive. That is why the source advises treating any dropped drive as damaged, unplugging it if still connected, and avoiding DIY fixes like tapping or freezing.
For businesses, the practical takeaway is to have a response plan: back up critical data, and if a drive is damaged, send it to a professional recovery service before attempting to power it on. If the data is not critical and you choose to test it, listen for abnormal noises and immediately copy important files if it mounts. But the safest route is professional evaluation.