When you’ve already rooted your Nexus one like me, you may noticed that even Google Push the update 2.2 to you device over OTA, you end up with a screen has a ! in the triangle.
Just Search “Install android 2.2 on rooted nexus one”, you may get tens of blogs.
I just follow on link here, http://www.redmondpie.com/how-to-install-android-2.2-froyo-on-nexus-one-with-root-9140788/
when I hold power/volum+ to the recovery mode, I don’t have the chose of Flash zip from SDK as mentationed in the blog
I only have 4 choices. as bellow
then I figure out we need to install recovery-RA-nexus which has the flash zip from sdcard features.
when you download the recovery-RA-nexus-xx.img, you need a utility called flash_image to load the imge to recovery partition. http://android.modaco.com/content/google-nexus-one-nexusone-modaco-com/299241/24-feb-1-6-2-ra-nexus-recovery-image/When I try this , it looks like there is no flash_image command in my system.
Finally , here is one step to port the utility to phone devices.
Download the flash_image.zip and unzip to your PC folder. install the ADB from Android Sdk site. then you will be able to run adb command
then run “reboot recovery” , the phone will be rebooted to recoery mode. you will be able to run the option called “Flash zip from sd card”
Hope you can get the following screen like me.