Most cheap SATA disk drives are rated with "1 non-recoverable read error per 10^14 bits read".
What does this mean?
10^14 bits is just 12.5 TByte. If I have a full 2 TByte disk and I copy it to a second disk, is there in fact a chance of roughly 1/6 that one of the files is corrupted?
If this happens, will the affected block be marked and reallocated? I think so because if the read would be successful on retry, it's not a non-recoverable read error.
However I am using lots of these disk drives for a couple of years now, I haven't noticed any increase of bad blocks count, also the RAID controller logs do not show any read problems.
EDIT: The RAID controllers do a weekly patrol read of each disk, so that amounts to about 100 TByte per year. That is still less than 10^15 bits.
On the other hand there were 4 total disk failures out of 50 disks within 2 years which increases the error rate.
I don't have enough statistically significant data to make statements but in my case the actual error rate seems to be between 10^14 and 10^15 which is consistent with the specification.