"adb devices" successfully shows device, but "adb connect usb" inevitably fails, any suggestions? I have an ASUS ThinkPad, Android 4.4.2, Kernel 3.10.20.  I've checked Settings->Developer Options->USB debugging.  I've disabled Wi-Fi and BT (because I've seen some devices have some conflict there).  Now here's the command line sequence from my linux desktop trying to connect to my ASUS:

joe@OptiPlex-755:~$ adb kill-server
joe@OptiPlex-755:~$ adb usb
* daemon not running. starting it now on port 5037 *
* daemon started successfully *
already in USB mode
joe@OptiPlex-755:~$ adb devices -l
List of devices attached 
E6OKCY917549           device usb:1-2 product:WW_K011 model:K011 device:K011

joe@OptiPlex-755:~$ adb get-state
device
joe@OptiPlex-755:~$ adb connect usb
unable to connect to usb:5555
joe@OptiPlex-755:~$
Here is the documentation on usage of the adb utility.

Basically, adb connect is only needed if you are trying to connect to a device over TCP IP, i.e. wireless network.  You don't need to issue any "connect" commands for a device connected via USB if it is already recognized in adb devices prompt.  Just issue any other command, e.g. adb shell ls /sdcard/ and it should connect.