controlling a machine with usb boot or target mode I need to change some preferences on a mac of which statup disk I can't control due to video issue (I don't see the desktop and so on ), I can use an usb disk to boot or launch it as target disk.... but can I operate on hits preferences?
thanks for now
If you boot your Mac into Target Disk Mode, you can access all of its files (provided you have the pertinent credentials) using another Mac. To start, reboot while holding down the T key on the Mac of which you would like to access the disk. 

Once booted into Target Disk Mode, connect it to another Mac. This can be done using a FireWire or Thunderbolt cable, or using a USB Type C > USB Type A cable if you have the MacBook (Retina 12") (note that Apple does not sell this cable, but Belkin will soon. You can use your standard USB Type C cable if you're connecting it to another MacBook). 

Then, to get to your displays preferences, open the property list located at ~/Library/Preferences/ByHost/com.apple.windowserver.&lt;uuid&gt;.plist. It's binary plist, meaning that you must either open it using a tool like Xcode, or convert it to XML by running plutil -convert xml1 com.apple.windowserver.&lt;your_uuid&gt;.plist on the command line in the appropriate directory to make it suit for opening by text editor. (Note that after this you will need to convert back to binary using plutil -convert binary1 com.apple.windowserver.&lt;your_uuid&gt;.plist).

When it's all said and done, you should be able to change your display settings back to ones appropriate for your display.