The problem is that taking some time to read the data doesn't mean the drive has gone bad, at worst perhaps a sector has gone bad, and even then, it might even

The problem is that taking some time to read the data doesn't mean the drive has gone bad, at worst perhaps a sector has gone bad, and even then, it might even still be readable, just a bit slow. And apparently it is very common on large capacity green disks after a few months. The typical bad scenario (and I have been burned by this) is that let's say you use RAID5, and one sector goes bad. While the disk tries to read it, the RAID controller kicks the disk out of the array because of the timeout. Now you need to replace the disk and rebuild the array. In the rebuilt, as you are doing a full read or all disks, you are pretty likely to find another sector that is slow to read on one of the other disks (particularly on multi-TB disks). And then the controller will kick another disk from the array. And now you lost everything. Also why data scrubbing is pretty important in NAS.