We were finding that using these NAS units showed some poor disk speed performance when we were running backups to the unit. After some work on the unit, one of the guys (with a ton more linux knowledge than myself) found that disabling the command queuing helped. His notes below:
ata1.00: configured for UDMA/XXX
exception Emask 0x10 SAct 0x7 SErr 0x400100 action 0x2 frozen
#!/bin/sh
echo 2 > /sys/block/sda/device/queue_depth
echo 2 > /sys/block/sdb/device/queue_depth
echo 2 > /sys/block/sdc/device/queue_depth
echo 2 > /sys/block/sdd/device/queue_depth