Is disk really bad if mkfs.ext4 -c -c reports errors?

[compdoc]

>Which says it's OK, but it doesn't appear to be OK.

There are a couple of problems with SMART that you should be aware of:

1) Most drives have a 'threshold' number, and will report the drive as
'healthy' until that threshold is reached. For example, if the threshold is
10 reallocated sectors, (bad sectors) and you only have a couple of bad
sectors, then the drive gets reported as healthy. But you have to ignore
that and read the actual number of bad sectors. (reallocated and pending
sectors) Some people ignore it if there are only one or two bad sectors, but
I'm in the business of keeping computers running 24/7 and I never ignore it.
I replace the drive. 

2) As far as I know, SMART doesn't predict mechanical failures. A mechanical
failure can happen at any moment, and when it does it's likely going to be a
catastrophic event. Besides SMART, the only other tool I use when diagnosing
drives is listening to it. If it's continually making odd noises, then a
mechanical failure is likely. 

Good luck, and be sure to back it up!