Jan 18 12:37:23 Tatooine preclear_disk_S3YJNS0N704931R[7562]: Signature: verifying unRAID's signature on the MBR ...
Jan 18 12:37:23 Tatooine preclear_disk_S3YJNS0N704931R[7562]: Signature: Unraid preclear signature is valid!
Jan 18 12:37:24 Tatooine preclear_disk_S3YJNS0N704931R[7562]: Post-Read: post-read verification started (1/5)....
Jan 18 12:37:24 Tatooine preclear_disk_S3YJNS0N704931R[7562]: Post-Read: verifying the beginning of the disk.
Jan 18 12:37:24 Tatooine preclear_disk_S3YJNS0N704931R[7562]: Post-Read: cmp /tmp/.preclear/sdb/fifo /dev/zero
Jan 18 12:37:24 Tatooine preclear_disk_S3YJNS0N704931R[7562]: Post-Read: dd if=/dev/sdb of=/tmp/.preclear/sdb/fifo count=2096640 skip=512 iflag=nocache,count_bytes,skip_bytes
Jan 18 12:37:25 Tatooine preclear_disk_S3YJNS0N704931R[7562]: Post-Read: fail - beginning of the disk not zeroed
This time the MBR was valid but the disk wasn't correctly zeroed, or is now returning non zeros, since it still happens with a different device it points to a hardware problem, like bad RAM, controller, etc.