When studying hard drive's documentation provided by vendors you can easily note that often manufacturers provide, to put it mildly, not real URE (Unrecoverable Read Error) values.
This URE specification is commonly applied to support bogus conclusions like "RAID 5 is dead" or to get chances of double read error in RAID5. Problem is, the vendor values are very far off the mark.
Read documentation on Hitachi website, where very interesting URE values are given, e.g. for 3 TB disk - 10-14 errors per bit read. Let's assume that this value is real. Thus, if you buy the given disk and start reading data off it from the beginning to the end then the probability not to encounter a read error is :
(1- 10-14)(8*3*1012)~0,79
so the probability that the hard disk wouldn't be able to read one sector is about 20 percent.
All the above means that when you are dealing with a disk completely filled there is a 20% chance that you are not be able to read data off it. This is obviously demonstrated wrong by everyday usage.
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