When ADB works but FastBoot does not…
I faced a very embarassing situation: trying to root my Nexus S, I had to unlock the device. My Android SDK was succesfully installed, the drivers too. The device was succesfully recognized by ADB… But fastboot devices failed to detect the device. From this point, it was impossible to unlock the device by launching fastboot oem unlock. I tried many basic solutions, but none worked.
At last, I found this post:
http://forum.xda-developers.com/showthread.php?t=875580
When ADB works but not FastBoot, the solution is to… install PdaNet. Amazing ; but efficient.
Many thanks to BigRick10 from XDA forums!