I just made an android app and I went to put it on my phone and This pops up about halfway through the process “Could’t find Android Device.
No Android Devices found. If you are sure that device
is attached then it might be USB driver problem, for
details please check Android SDK Setup section in manual.”
PLEASE HELP QUICK !!! 
Make sure you have the Android SDK drivers installed, the phone USB drivers installed, and your phone has USB debugging enabled.
I will see if I am missing any drivers. I thought I had everything Thanks for the reply 
Additional to that, make sure there is no Device Software active. If you have an HTC Phone and HTC Sync installed, you must first quit HTC sync. Some PC software for smartphones comes with their own adb.exe versions and this can be a cause Unity can’t find the device
Does a mouse count ??? My mouse is always plugged in, I have a laptop.
I had a problem once after a JDK update but I think this has been solved. Everthing runs fine for me (Nexus S, Xoom, Kindle Fire) with JDK 6 Update 29.
I just disabled the debugging mode in my phone, and enabled it again. On Windows, the USB driver has conflicts connecting with the phone if you connect it and disconnect it many times.
@myka: I take it you are you trying to run it with an emulator and not an actual device? If so, do you have a valid emulator running with the AVD Manager? Just checking.
same error message here…only mine is on mac…what to do?
Just fought with this for quite a while. It was most frustrating because everything has been working for a long time until today.
Finally figured out I had updated my phone last week, but had not downloaded the new android SDK for that version. It wasn’t the version I was building to, but Unity needed that version of the SDK installed to see that it was plugged in. Installed the new version through the Android SDK Manager and everything was fine. (Another note, I had to update the SDK tools and some other package before the manager would let me install the new SDKs.)
Just leaving a note here to help others, as I came across this on my way.
I also simply disconnected and reconnected the device. It worked fine.
On a mac.
Thanks for suggesting this, I had the same issue today. I have never had an issue until today out of the blue. I ran the update on the USB driver from google, alas I Still have the issue since windows can’t seem to find the driver.