Q: Cause of android adb "usb device not found" problems? I've found a variety of questions here talking about solutions to the "usb device not found" problem when using adb, but I haven't found any explanations about what's going on or if there's a long term fix.
When I'm developing I'll write some code then hit run or debug, watch the results on my device then repeat the process.
Every now and then, perhaps once every few days, I'll get the "usb device not found" error.
The first thing I try is to open up the task list and kill adb.exe. That sometimes works.
If that doesn't work the next thing I try is to kill adb.exe then disconnect the android device, reboot it, then reconnect it and then run "adb usb" from the command line. That almost always works.
If the above doesn't work then I'll reboot the android device and also reboot the computer. That seems to do it for all the remaining times that the problem occurs.
Does anyone know why this happens and if there's a way to prevent it from happening? My only theory (based on not much) is that perhaps the device gets hot (due to heavy cpu usage during development and debugging) and does something wrong which messes with the way adb does things.

A: I had the same problem. I ended up having to go to  my computer/properties/advanced system settings/environment variables/ , click "PATH" , then paste ";c:\Android\tools;c:\Android\platform-tools" as the "variable value" 
then i went into device manager - my computer/properties/device manager and seeing that the droid had an "!" by it. I then downloaded and unzipped android ADB into a file on the desktop. and then back in device manager installed the driver by right clicking droid/then clicking update driver software , and selecting the file on the desktop and it, voila.
it took 20 seconds.
