Loss prevention
The most important part of preventing data loss is to keep two or three copies of your data at all times. When it comes to mechanical devices like hard drives, it is not a question of if it will fail but a question of when it will fail. Use protective software such as firewall's and virus scanners to prevent malware from reaching your data. Make sure your drives are cooled properly. Use quality power supplies and controllers.
Note: Dying drives have only so much life left. Powering on a hard drive with hardware problems can do further damage and make your data permanently inaccessible.