# show sync status
cat /sys/block/md0/md/sync_action

# show sync progress
cat /proc/mdstat

# show details
mdadm --detail /dev/md0

# mark faulty
mdadm /dev/md0 -f /dev/sdc1

# remove drive
mdadm /dev/md0 -r /dev/sdc1

# readd drive
mdadm /dev/md0 -a /dev/sdc1