Backup using ADB without root or usb debugging I have a Nexus S (on android 4, not rooted) device who's not able to boot. It opens and get stucks to the X screen. I can only access the recovery mode.

I tried to backup data via ADB, but the phone haven't activate USB debugging so I can't connect ADB backup (and it won't show in adb devices). I can only get signal via USB when I go in "update via ADB" option, then the device show, but only the sideload connection is activated.

How can I backup the data on the phone? Or how could I activate the USB debugging mode while in recovery?
You may need to 'upgrade' the driver to the adb version.
Open Device Manager, find your phone and right click it. select "update driver software". On the window that appears choose the "let me pick" option. You should then see a couple of different drivers, select the one that includes "adb" and click ok through any further windows. (remember which one was selected before so you can repeat this to return to the 'normal' driver)
You should then be able to run the adb backup.