On some secondary GPUs they report that they have the VGA port enabled. This has been seen on the GeForce GT 520M.
Symptoms include higher drain on the battery, fans constantly running, Xorg attempting to treat the discrete card as a secondary monitor.
This issue can be detected by opening a console and typing xrandr:
[root@localhost ~]# xrandr
Screen 0: minimum 8 x 8, current 2390 x 768, maximum 32767 x 32767
LVDS1 connected primary 1366x768+0+0 (normal left inverted right x axis y axis) 293mm x 164mm
1366x768 60.02*+
1024x768 60.00
800x600 60.32 56.25
640x480 59.94
DP1 disconnected (normal left inverted right x axis y axis)
HDMI1 disconnected (normal left inverted right x axis y axis)
VGA1 disconnected (normal left inverted right x axis y axis)
VIRTUAL1 disconnected (normal left inverted right x axis y axis)
VGA-1-2 connected 1024x768+1366+0 (normal left inverted right x axis y axis) 0mm x 0mm
1024x768 60.00*
800x600 60.32 56.25
848x480 60.00
640x480 59.94
1024x768 (0x63) 65.000MHz
h: width 1024 start 1048 end 1184 total 1344 skew 0 clock 48.36KHz
v: height 768 start 771 end 777 total 806 clock 60.00Hz
800x600 (0x64) 40.000MHz
h: width 800 start 840 end 968 total 1056 skew 0 clock 37.88KHz
v: height 600 start 601 end 605 total 628 clock 60.32Hz
800x600 (0x65) 36.000MHz
h: width 800 start 824 end 896 total 1024 skew 0 clock 35.16KHz
v: height 600 start 601 end 603 total 625 clock 56.25Hz
In this instance you can see there is an LVDS1 device which is your normal screen and GPU. We also have a VGA1 which is the VGA port on the laptop, and VGA-1-2. This is the VGA port that the discrete GPU is reporting it has.
To get your machine working to it's optimum we need to disable that VGA1-2 port. This has to be accomplished by setting video=VGA-2:d as a kernel flag.
This can be done manually in grub at boot time (press e) or made perminent by appending to the line GRUB_CMDLINE_LINUX in the /etc/default/grub file and updating your grub config.
Be aware that in some instances this could mean your external VGA port may no-longer work. This is because it is possible that the discrete GPU is connected directly to your VGA port on your laptop or used as a pass through.