Many computer components have practically limitless aspects you could theoretically measure. For example, for a hard drive, you could measure the capacity, seek time, read rate, write rate, failure rate, weight, warranty, and customer satisfaction rates - just to name a few.
I think your posts might get more responses if you could ask more specific questions.
Think of it this way - if someone asked, "Explain everything about cars and pick one for me," you might not know what to say.
As far as hard drives go specifically, I would ask you what your priorities are among the following three issues: high speed, large capacity, and low price. With current market conditions, a very rough estimate would be to say that you can have any two of those three things, but probably not all three.