Reminder
This helped me to fix my NAS hanging regularly on copying big files.
system-wise
echo 1024 > /sys/block/sda/queue/max_sectors_kb
blockdev --setra 65536 /dev/sda
echo deadline > /sys/block/sda/queue/scheduler
samba-wise
oplocks = no
level2 oplocks = no
some notice using FTP copying process seems to work much more reliable. Haven't tried that though.
and this one http://community.wdc.com/t5/My-Book-World-Edition/MyBook-keeps-crashing-when-copying-large-files/td-p/10204/page/2
looks like other folks are having same issues... ugoh.