The recovery of bad sectors can be two fold.... the older drives could experience variance and creep over time... slowly the sectors creep forward or backward on the physical surface of the disk, sometimes remnants get in the way other times there is an actual defect there when you low level it may miss the defect completely or unify good remnants making it whole again as the two cells now become one.
It was good to periodically refresh a drive and write all the data out again, performing a verify a time or two days apart before erasing original data. Multiple low levels can allow the drive to drift past a bad spot and allow for better results... less or no bad sectors.. generally it will be fine as most drives drift in the same direction so once it is at it's best point you can run with it..
There were two schools of thought, low level till you hit the most bad sectors and map them out, if you drift you won't hit the bad spot anyway.. or low level until you have the least and enjoy the added space putting your most read and least written stuff in that area... it may drift and come up bad later but since they normally drift in the same direction, if you write very little you may never hit it again, if you write to that area a good amount it will eventually hit it again any way no matter what you do..
- DavidMil likes this