подобрал смещение и длину, при которой битый диапазон перекрывается:# /bin/dd if=/dev/ad4 of=/dev/ad5 bs=512 iseek=12000000 oseek=12000000 count=1024000 conv=noerror,sync
kernel: ad4: FAILURE - READ_DMA status=51<READY,DSC,ERROR> error=40<UNCORRECTABLE> LBA=12702677
kernel: ad4: TIMEOUT - READ_DMA retrying (2 retries left) LBA=12702678
kernel: ad4: TIMEOUT - READ_DMA retrying (1 retry left) LBA=12702678
kernel: ad4: FAILURE - READ_DMA timed out
kernel: ad4: TIMEOUT - READ_DMA retrying (2 retries left) LBA=12702680
kernel: ad4: TIMEOUT - READ_DMA retrying (1 retry left) LBA=12702680
kernel: ad4: FAILURE - READ_DMA timed out
kernel: ad4: TIMEOUT - READ_DMA retrying (2 retries left) LBA=12702682
kernel: ad4: FAILURE - READ_DMA status=51<READY,DSC,ERROR> error=1<ILLEGAL_LENGTH> LBA=12702682
kernel: ad4: FAILURE - READ_DMA status=51<READY,DSC,ERROR> error=1<ILLEGAL_LENGTH> LBA=12702683
kernel: ad4: TIMEOUT - READ_DMA retrying (2 retries left) LBA=12702684
kernel: ad4: TIMEOUT - READ_DMA retrying (1 retry left) LBA=12702684
kernel: ad4: FAILURE - READ_DMA timed out
kernel: ad4: TIMEOUT - READ_DMA retrying (2 retries left) LBA=12702686
kernel: ad4: TIMEOUT - READ_DMA retrying (1 retry left) LBA=12702686
kernel: ad4: FAILURE - READ_DMA timed out
kernel: ad4: TIMEOUT - READ_DMA retrying (2 retries left) LBA=12702688
kernel: ad4: TIMEOUT - READ_DMA retrying (1 retry left) LBA=12702688
kernel: ad4: FAILURE - READ_DMA timed out
kernel: ad4: FAILURE - READ_DMA status=51<READY,DSC,ERROR> error=1<ILLEGAL_LENGTH> LBA=12702690
Итого, не так уж и много битых...
12702677
12702678
12702680
12702682
12702684
12702686
12702688
12702690