Official comment
With the library on Computer A, take the USB drive and connect it to Computer B, export whatever you want to migrate to the USB drive, and then connect it to Computer A. You'll see it appear as a device. Select all the tracks in the device's collection and right-click > Import. They'll then be added to Computer A.
From there, your best bet is to move the database and music to an external drive which you could then move between computers. Instructions for that are here.