What are attributes, values and thresholds? 

 An attribute is something that a specific hard disk logic is able to analyze and report about. Every hard disk can include a different set of attributes. Every device manufacturer can publish some attribute based on its ability to report about it and on his knowledge that such an attribute is useful to decide about hard disk reliability over time. Every attribute can assume a value. Such values change over time. Higher values indicate a better health, while lower ones should be considered symptoms of something that either degraded or is degrading. Every attribute has a corresponding threshold. When an attribute value is the same as or lower than its threshold, the drive is considered to be failing S.M.A.R.T. status. A threshold of 255 means something that will alway fail and should only be used for test purposes. A threshold can be only be set to values from 1 to 253. 254 is forbidden and 0 means that the attribute the threshold is associated to should be considered only informational and that it has no direct influence over reliability. Every attribute stores the worst value it ever assumed and some raw data. Raw data is highly vendor specific and only specific tools find it really useful. It should be noted that some kind of silent agreement exists over raw values and some kind of standard might be assumed. According to S.M.A.R.T. specifications, no linearity is implied in attribute values, but since lower values mean worst conditions, even if linearity is not mandatory, it can be assumed as a first guess. It should be noted that a lot of things in S.M.A.R.T. specifications are left up to