CPU Fan stuck on 4218 rpm

OK, so SSH into the NAS and look at /etc/sensor.d and see if i you have both a system.conf and a pro6-fan.conf.

pro6-fan.conf should look like this:

chip "w83627dhg-*"
        ignore fan3

And system.conf like this:

chip "w83627dhg-isa-*"
        label in1 "DDB18"
        set in1_min 1.62
        set in1_max 1.98
        label in2 "AVCC"
        set in2_min 3.3 * 0.90
        set in2_max 3.3 * 1.10
        label in3 VCC3
        set in3_min 3.3 * 0.90
        set in3_max 3.3 * 1.10
        label in4 VCC1_25
        set in4_min 1.17
        set in4_max 1.32
        label in5 "VCC1_5"
        set in5_min 1.41
        set in5_max 1.58
        label in6 "VCC1_05"
        set in6_min 0.98
        set in6_max 1.10
        set in7_min 3.3 * 0.90
        set in7_max 3.3 * 1.10
        set in8_min 3.0 * 0.90
        set in8_max 3.3 * 1.10
        label fan1 "System"
        set fan1_min 600
        label fan2 "CPU"
        set fan2_min 150
        label fan3 "System"
        set fan3_min 600
        ignore fan4
        ignore fan5
        label temp1 "System"
        set temp1_max 80
        ignore temp2
        ignore temp3
        ignore cpu0_vid
        ignore intrusion0

system.conf is actually just a link to etc/frontview/sensors/NV6.conf.

I'd say your fan or the fan controller was broken, but that would normally make it start out full on, not ramp up.