readynas ultra6 fan speed

OK, PWM2 is the controller the OS is trying to use, as your config file shows.  But with a setting of 138, I would expect the fan to be going faster.  But each system has a range, and that may be right.  It's also odd that it seems it may also be changing PWM3, which is what I thought it would do based on the ULTRA6.conf file in the OS on my NAS (though my NAS is not an Ultra6, there are copies of all config files so you can swap the array to another model).  So, try this:

echo 1 >pwm2_enable
echo 255 >pwm2

You may need to wait a few seconds between them.  Try them twice before you give up.  The first command changes the fan control to manual, and the second sets the fan speed to max.  If your fan doesn't start really moving, then try spinning up fan 3 to max:

echo 255 >pwm3

Whatever happens, don't let it stay that way:

echo 140 >pwm2
echo 140 >pwm3
echo 3 >pwm2_enable

If the manual control of PWM2 did spin the fan up, try switching the fan mode in the GUI and see if it now has any effect on the actual fan speed or the pwm2 or pwm3 registers.  If the change of PWM3 worked, then your config file is wrong.

If it didn't change with either, do a cat pwm2_mode pwm3_mode.  A zero is DC output control for an analog three-pin fan and a 1 is PWM control for a 4-pin one.  I don't recall which the Ultra6 has, but you can try flipping those mode registers by doing a similar echo command to each.

Rather than me going through all the possible outcomes and what they might indicate, report back what you find.  Hopefully, the SPAM filter will be kinder to you this time.

Also, if you or anyone else knows which fan type the Ultra6 uses and if, like the Pro6 CPU fan, it has provisions for either one via different connectors, that will help.  Also, do you know if the unit has ever been opened and the fan may have been moved to the wrong connector?

If I have to, I'll retrieve my Ultra6 from my storage unit and take a look at the hardware and config file.  But don't count on me doing that quickly.