When does an HDD replace bad sectors?

Giganews Newsgroups
Subject: When does an HDD replace bad sectors?
Posted by:  pimpom (pimpom@invalid.invalid)
Date: Thu, 21 May 2009

I understand that a modern HDD has many spare sectors which it automatically
substitues for bad sectors until it runs out of spares, and that a bad
sector showing up in the OS is a sign that the drive has aged too much to be
reliable.

But when does the substitution take place? I expect that, for one, it
happens during a full format. Does it also happen quietly in the background
on the fly during normal use under an OS?

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