Unable to check Turtlebot Cable

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hello, when I try to run

roslaunch turtlebot_bringup minimal.launch

this is error appear : Failed to open port /dev/ttyUSB0. Please make sure the Create cable is plugged into the computer.

we try to solve this problem ,we search alot in the Community So, we add this ATTRS{idProduct}=="2303",ATTRS{idVendor}=="067b",MODE="666",GROUP="turtlebot"

in this file :/etc/udev/rules.d/52-turtlebot.rules

this file created filesystem/dev/ttyUSB0 after :

roslaunch turtlebot_bringup minimal.launch

But after :

roslaunch turtlebot_navigation gmapping_demo.launc this file created :filesystem/dev/ttyUSB1 and USB0 disappear and this error appear again unable to check cable.

,I'm using fuerte on ubuntu 11.10.

Any help??