How to diagnose a failing HDD

HDDsaver.com shows us howto diagnose failing hard drives:

Many times before a hard drive goes completely bad there are signs foreshadowing its demise. If these signs are observed and recognized there is a good chance that the data on the drive can be saved. The window of time in which action any will be useful is quite small but if the proper precautions are taken there is a chance than a real problem can be avoided..