psusi nails it.
If you read the data sheets (white papers) say at seagate.com
you will see how HDD's are made, tested and how they really work.
There is no perfect HDD, never was, never will be, (history and fact).
In the olden days, we had to enter the bad sectors in to the HDD controller
from a list on paper that came in the new drive box, so the controller skips them.
Modern drives have error correction. From day 1 sectors are bad.
So they map them out, this means the drive skips bad sectors.
In fact they are "logically swapped out" - the bad sector is mapped to a new, good, spare cylinder sector (it has spare cylinders - think of cylinders as tracks). This is all transparent to the outside world - except for the SMART util.
Each manufacturer can do as they please, so some set the error counts to zero, even though there might be 10 bad sectors as soon as the drive is manufactured.
There is a 3 times rule in the drive's firmware - it reads a sector 3 times and if all 3 times it is bad then it may do a "recalibrate" on the fly, and read 3 more times. If the drive is still not ok it will map that sector to one of the spare sectors. This is deep in the firmware, but happens continually in the background, all transparent to the user.
Whether the manufacturer chooses to report raw errors whenever there are 3 bad reads or after the calibrate is up to them. So like he says above, it's not important unless you have many drives of the same kind and you see some odd trends.
Point 2: all HDD have natural read errors, you can learn that at Seagate too, if you want. but they all have errors on the fly. and are read again, and usually pass the test for CRC errors. if not the DRIVE tries to swap it out.
if you run the disk cool, it will last a long time and you many never run out of spare cylinders. but do look at that as psusi tells you !
I'm typing this , on an old PC , running one of the first 1gb HDD ever made. and is still good. (im backed up ) (no lack of cooling ever...) heat is the #1 killer and power surges, I run a UPS. cheers and good day. I hope this helps.
(ever seen a DatA General hard disk crash? and fill the room with vast amounts of aluminum wool, curly cues? lots of fun back then... never a dull moment....