dd if=/dev/sda of=/dev/null

One of the two 6 TB hard drives on my home server is claiming that it has some sectors it can no longer read. That normally means that there is damage on the surface. The firmware on the drive should mark them bad and remap the data to some spare ones. As long as it doesn't run out of spares, it will be fine and there won't be any data loss. This generally isn't a good sign for the life of the drive... I tried to read the disk to see if I could force the firmware to reallocate the sectors, but it takes a long while to read the whole of a 6 TB disk (~2 days) and I've still not forced it to reallocate them yet...