Then chkdsk and the like will start to report bad sectors. The entire issue is well documented so I'll stop here and wait to read why you asked.
Most of todays HD's have the ability to auto spare a defective sector.
If this activity takes place and I run a product like scandisk can I see a sector marked bad?
I get the impression the answer is NO.....all sectors will show good.
If the auto spare activity takes place and all the spare sectors for that track are in use.....what happens?