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The Crazyflie 2.X ID is a character string of 24 characters. It is unique to each Crazyflie 2.X.
The serial number is located just after “5740\”, in the following screen it starts by 41:
By right-clicking on the string you can copy it.
Open a terminal and type (ie. cop-paste) the following command:
sudo lsusb -v -d 0483:5740 | grep iSerial
The Crazyflie serial is then displayed (here it starts by 41):