Uncorrectable sector count 1, pending sectors 0, reallocated 0 on two drives for years - why uncorrectable sectors were not reallocated?

Question

I have two WD Blue 1TB drives since year 2015, bought new from two different shops, used in two different machines. The disks have different manufacture dates. I haven't yet experienced any issues with them - no slowdowns, no data corruption (although I haven't tested all the files on them, so there's a chance that one of them might be corrupted).

Full non-destructive scanning with Victoria and HD Tune doesn't report anything critical nor warnings. Still, some disk utilities have marked the disks as suspicious because of Uncorrectable Sectors.

Here are the relevant SMART values. Both disks have the same values:

ID Cur Wor Thr RawValues(6) Attribute Name
05 200 200 140 000000000000 Reallocated Sectors Count
C4 200 200 __0 000000000000 Reallocation Event Count
C5 200 200 __0 000000000000 Current Pending Sector Count
C6 200 200 __0 000000000001 Uncorrectable Sector Count
C8 200 200 __0 000000000001 Write Error Rate

As you can see, there have been no sector reallocations and there are no pending sectors.

What does the Uncorrectable Sector Count mean in this case?

If there was an error while writing (as Write Error Rate suggests) and a sector was marked as uncorrectable, why didn't the drives attempt to reallocate the bad sectors? Or does it mean just that an error occurred once (maybe during an abrupt shutdown during a power failure) and then later the same sectors could be written again... but then why call them "uncorrectable" if the situation is normal again?

I have tested the situation after half a year - still the same, nothing gets worse. Still curious, how exactly this situation should be interpreted.

Answer 1

What does the Uncorrectable Sector Count mean in this case?

Your disk is probably safe, but read on ...

Reported Uncorrectable Errors S.M.A.R.T. parameter indicates a number of errors that could not be recovered using hardware ECC (error-correcting code).

Recommendations

Although this parameter is not considered critical by the most hardware vendors, degradation of this parameter may indicate electromechanical problems of the disk. Regular backup is recommended. If no other (critical) parameters report a problem, hardware replacement is recommended on mission critical systems only.

Source S.M.A.R.T. Attribute: Reported Uncorrectable Errors | Knowledge Base