The drive keeps trying to read what's in there and then , relocates it to a good sector but what if it can't read what is in there ?
Then those sectors will be marked as pending sectors . So if you DON'T need what's there , you can tell the HDD that you don't even care what's there and it can stop trying to read that sector and relocate it somewhere else . BUT how ? You can perform a full format ( not a quick one ) and them hard drive will realise that it can remap that sector to somewhere else but since you need your info and can't perform a format , You can use softwares as Western Digital Data Lifeguard