外接屏幕已被检测到但一直黑屏

[ssiaw12345]

Ubuntu16.04 笔记本电脑
Nvidia 970M+Intel双显卡，已更新到最新驱动
外接屏幕刚接上时显示正常，外接屏幕的分辨率为1440x900.由于我想将外界屏幕竖起来使用，在系统设置-显示-方向那里只有正常，而没有其他方向可选，于是用了命令xrandr -o left后，外接屏幕黑屏，主屏幕只有一小块有显示。于是重启。
重启后外接屏幕黑屏，但屏幕是检测得到的，只有选择1280x1024的分辨率，外接屏幕才不黑屏。
这是我运行xrandr后的信息：
Screen 0: minimum 8 x 8, current 3360 x 1080, maximum 16384 x 16384
eDP-1 connected primary 1920x1080+0+0 345mm x 194mm
1920x1080 60.02*+ 59.93
1680x1050 59.95 59.88
1600x1024 60.17
1400x1050 59.98
1280x1024 60.02
1440x900 59.89
1280x960 60.00
1360x768 59.80 59.96
1152x864 60.00
1024x768 60.04 60.00
960x720 60.00
928x696 60.05
896x672 60.01
960x600 60.00
960x540 59.99
800x600 60.00 60.32 56.25
840x525 60.01 59.88
800x512 60.17
700x525 59.98
640x512 60.02
720x450 59.89
640x480 60.00 59.94
680x384 59.80 59.96
576x432 60.06
512x384 60.00
400x300 60.32 56.34
320x240 60.05
VGA-1 disconnected
DP-1 disconnected
HDMI-1 disconnected
HDMI-2 connected 1440x900+1920+0 408mm x 255mm
1440x900 59.89*+ 74.98
1280x1024 75.02 60.02
1280x800 59.91
1152x864 75.00
1024x768 75.08 70.07 60.00
832x624 74.55
800x600 72.19 75.00 60.32 56.25
640x480 75.00 72.81 66.67 60.00
720x400 70.08
DP-2 disconnected
HDMI-3 disconnected

在得到上面信息时，外接屏幕是黑着的，但我鼠标呀窗口啥的可以移过去。
这是我/etc/X11/xorg.conf的内容：
Section "ServerLayout"
Identifier "layout"
Screen 0 "nvidia"
Inactive "intel"
EndSection

Section "Device"
Identifier "intel"
Driver "modesetting"
BusID "PCI:0@0:2:0"
Option "AccelMethod" "None"
EndSection

Section "Screen"
Identifier "intel"
Device "intel"
EndSection

Section "Device"
Identifier "nvidia"
Driver "nvidia"
BusID "PCI:1@0:0:0"
Option "ConstrainCursor" "off"
EndSection

Section "Screen"
Identifier "nvidia"
Device "nvidia"
Option "AllowEmptyInitialConfiguration" "on"
Option "IgnoreDisplayDevices" "CRT"
EndSection

在线等帮助，如果可以的话，顺便求选装屏幕方向的方法,谢谢！！！

[ssiaw12345]

补充：之前使用的是HDMI转DVI连接线，现在测试了HDMI转VGA线，显示正常了，现在求解如何旋转屏幕方向，谢谢！