Based on the main causes of data loss described previously, these are the most important steps you can take to protect data on an external hard drive:
Keep the drive away from excessive heat or moisture. Keep your external drive out of the sunlight and out of hot parked cars.
Always maintain strong antivirus protection with a quality commercial application. Periodically running a scan on the external drive is a good idea to keep it free of malware.
Take care when deleting files or formatting partitions. Make sure you have specified the target correctly. Check your work before pressing that button!