Depends on the case. Facebook at least generally replaces drives as they go. In a pipelined fashion rather than as they fail, though. So, a given drive failu

Depends on the case. Facebook at least generally replaces drives as they go. In a pipelined fashion rather than as they fail, though. So, a given drive failure doesn't lead to immediate emergency replacement, but sometime in the next few days as someone makes a drive-replacement pass through that area.