mounting google nexus s on linux at fastboot mode screen
To my knowledge, in fastboot mode you cannot mount the device to your PC. MTP and fastboot are two different things.

According to the CyanogenMod Wiki on Fastboot, the solution might be simple:


  If you see  at any point when executing fastboot command, the PC is not configured to use fastboot properly. Most typically, this is because you are not running fastboot with elevated privileges.


Which means: you must run fastboot as root user, e.g. sudo fastboot devices or sudo fastboot-linux oem unlock.



If that does not solve your issue, it might help to take a look at the steps necessary to use the adb command line utility: For adb to detect your device, you need to have setup a corresponding entry in your /etc/udev/rules.d. First run lsusb to check how the device identifies itself to the system. These data you can also take from your above quoted log: idVendor=18d1, idProduct=4e20 is the information we need. Now, the corresponding entry in /etc/udev/rules.d/51-android.rules should look like:

SUBSYSTEMS=="usb", ATTRS{idVendor}=="18d1", ATTRS{idProduct}=="4e20", MODE="0666" GROUP="androiddev", SYMLINK+="android%n"


Save the file, and reload the udev service (service udev reload). Re-connect your device. Now adb (or, in your case, fastboot) should be able to find it. You can check that using the command adb devices (or fastboot devices), which now should list your Nexus S.