If I am not sure of a drive I will install Hard Disk Sentinel and I can usually get enough information within the trial period to know if I am going to replace it or not. It can give you an estimate on remaining life as well as tell you how many sectors are consistently bad vs just a momentary issue. Also has the drive power on time stats and if given some time will have an average temperature graph. It recently saved me on some used sas drives that were sold as new.