Running Debian 7, mdadm 3.2.5. I issued a command to grow a 5 disk, 3 spare raid 5 array to an 8 disk, 0 spare raid 6 array:
mdadm --grow /dev/md0 --level=6 --raid-devices=8 --backup-file=/root/raid5backup
Between one and five seconds after the drives spin up, there is a little clikc sound and they stop, and a ton of error messages spit out to the console. Googling chunks from these messages (there's more every 1-5 seconds so I can't get it exactly) finally landed me with this libata error message breakdown page.
According to that linked page, my issue is "communications between device and host temporarily lost, but regained". Looks like a bus error - could it be a shoddy cable? This machine has 11 SATA devices, 6 through onboard connectors and 5 more split over two expansion cards. The motherboard is brand new.
I'm not sure what to do with the messages though. Should I shutdown and try replacing cables? Any ideas?