I ran hard disk checker, and i found this error:
Write Error Rate: number of errors while writing to disk(or) multi-zone error rate (or) flying-height --- Normalized: 200 Worst: 200 Value: 21
What type of errors is this?
And how is this fixed?
That's not an error. Its just a count (21) of some event. It gave you the normalized value (200), but not the threshold, so there isn't any way to tell if the HDD vendor considers this too many or not (but I'd guess no, because lower normalized values are bad). 'Worst' is misspelled too, makes me think your hard disk checker is seriously lacking in polish.
I'd suggest using