How to restart USB subsystem?
You can reset the USB Bus with the USB Prober.app.  It's not included with XCode, but you can download it separately from the Apple Developer Download page.  You will need to signup and login.


search for iousbfamily
Download the latest IOUSBFamily Log Release file (at the time of this post it was IOUSBFamiliy Log Release for OS X 10.9.3)
Mount the .dmg and navigate to the directory with IOUSB-XXXXXX-log.
DO NOT install.  Just right click the package and "Show Package Contents"
Unzip the Archive.pax.gz; the file USBProber.app will be in /Archive/DevTools/Hardware




Important:

Just launching the app will reset your USB bus.  Make sure you you unmount any drives you don't want unceremoniously and improperly removed!