Modern disks are like cd's they have layers and layers of ECC (both spinning and flash at this point) and a certain assumed BER. What you describe is the usual

Modern disks are like cd's they have layers and layers of ECC (both spinning and flash at this point) and a certain assumed BER. What you describe is the usual case with an unscrubbed drive. As it slowly bitrots everything appears just fine, until that day you happen to read a file on a part of the disk that hasn't been accessed in a couple years. The suddenly the controller's ECC can't correct the error and you take a read fault. Then you start listing the directory and discover that yah there are bad sectors everywhere. So, sure you can have catastrophic failure, but the usual case with a well scrubbed drive is a corrected/relocated error once in a while. As long as they remain intermittent your fine, if suddenly you get 100 errors on one scrub and then the next one gets another 100, you better replace that disk fast. Or the other case is that you go from just fine, to pretty much nothing can be read in a single scrub cycle. You don't really have a choice in that case, the controller will likely just kick it out.