This answer includes speculation and opinion, and is more fun than fact.
http://www.seagate.com/staticfiles/support/seatools/user%20guides/SeaToolsDOSguide.EN.pdf written around 2010
By design, modern disk drives maintain spare sectors for reallocation
purposes. Usually, sectors become difficult to read long before they
become impossible to read. In this situation the actual data bytes in
the sector are preserved and transferred to the new spare during a
sector reallocation. Similarly, when a disk drive writes data and
encounters a problem, the drive firmware retires the problem sector
and activates a replacement before giving successful write status.
From the logic of this any "real" new bad sector marked out by the hardware , may very well have had a "real" problem at one time or another , and should not be re-tested and re-used , because it is a problem waiting for a place to happen, just like the original marked out bad sectors.
The space consumed by the few "bad" items is so small usually it is not worth concidering using them.
Should a re-test utility test the sector under many conditions? Yes it should, why wouldnt it, it would take minor extra time to confirm that multiple and different writes to it is readable, and would not cut into the total in the area. Adding ,why not a whole bunch of sectors in that same location seeking if there was any actual damage in the area , or just a fluke at the time?
If you make an impact on the external drive, it may cause bad sectors
on the disk. Bad sectors can cause various and potentially read/write
The impact bad sectors always seem to show in groupings in a similar area, with added bad sectors growing from that. a person should be able to get an idea of actual head impact damages on surfaces from the grouping of them. Any actual physical damage to an area, that releaces any particals from that area, those particals have to be slung off and filtered out.
From all the other information, any head impact damage might also result in the head itself becomming worse? If there is noticable accumulations of new bad sectors in groups, I dont think I would want to rely on that drive, but my drives usually don't have a lot of (shown) new remapped sectors at all.
Power fluxuation, static induction, interferance , atomic size warble, temperature, the data density , I would wonder if the Hardware itself had a error with "some sectors" under "less than ideal" circumstances, that at any rate they would still be the "worst of the bunch". When The manufactures own tests log-out bad original sectors , do they go back and say "well the ac clicked on in the building, or we had a flux in background radiation, so lets re-test that?" :-) Or do they figure that "under Any condition" the rest of them worked this one did not?
If there was a way to determine if the sectors are damage, growing damage from suface damage or surface imperfections, it should be the grouping of the sectors marked bad. ARgggg
I think only a hard drive manufacture could correctaly answer this, there are other data sheets that go well beyond my skill levels. At seagate WD seems to stick more with the simpler.
The most I have to go on, is what I have seen and experieneced and the data they provide. There are times when the stuff is deemed bad, and I have re-used it, and it never presented a problem, I knew at the time it was a software/hardware problem of my own. If it was damage I caused I would hope it logged out all 15 tracks there to never pass by the whole area again :-). When looking inside the drives, it is beyond me, they are perfect, Following this picture we see The molecular mounds the head tries to fly over. And Magnetic force microscopy?
If a hard drive manufacture did know all the answers, then it would not explain all the angry users who recieve Re-Tested Re-manufactured drives as a replacement, and have problems with them. Some users indeed were the problem, but not all of them.
Follow this picture To see an idea of "weak heads"
0s or not , I doubt it matters a Write is very complete