A HDD only detects faulty sectors when it tries to read from it but fails. Then it adds the sector to the
pending sector count. Which will reallocate it to somewhere else as soon as you try to write to this sector the next time. (
reallocated sector count)
Why did the read fail? Either the writing failed (and was not noticed since HDDs just write and assume it was successful) or the reading simply does not work for whatever reasons. The thing is, to be sure that your HDD is okay you need to write to every sector and then read every sector.
If after reading every sector the
pending sector count has increased, you know that something is still broken.
I would suggest a tool like http://hddscan.com/
Use it to write and read every sector (preferably several times) to ensure your drive is okay (i.e., the yellow values do not change). Also the
pending sector count should be zero.
Nevertheless you should probably backup and replace the drive. ,
After their first reallocation, drives are over 14 times more likely to fail within 60 days than drives without reallocation counts, making the critical threshold for this parameter also one.
Source: Failure Trends in a Large Disk Drive Population