Remove the USB drive, if the icon is showing on the desktop right click it and select Eject.
Once removed, clicks the Apple Logo in the top left hand corner and select Force Quit.
From the Force Quit window, click Finder, the button in the lower right hand corner will say Relaunch. click Relaunch and Finder (your desktop environment) will reload itself.
Once done, reconnect your USB drive and it should be recognised.
If it isn't, remove the drive and restart your machine, but reset the PRAM before it restarts, by holding down the R, P, Alt (option) and Ctl keys together (it's a two hand job, Alt + R with left hand Ctl + P with the right hand) keep holding all keys, the start up sound will sound, your system will reboot again, the start up sound will be louder the second time.
Depending on which version of the OS you have installed, you can release the keys after the second start up sound or keep holding them down until you hear a third start up sound. Then you can release the keys and your system will restart normally.
Once the desktop has loaded, connect your device and see if it's recognised.