The general rule of thumb is to use the rear USB because people in the past have been known to have issues with the front USB.
In terms of your box not recognising the USB when flashing, you have 2 options:
Option 1: go to your nearest store that sells memory sticks and pick up a 8gb Sandisk Cruze blade, these work straight out of the packet. If you can't get the 8gb then go for the 16g as this also works straight out the packet.
Option 2: download hdguru and do a low level format of the memory stick you already have.
Once you have done one of the above basically unzip your file that you downloaded to flash your bix with> drag the file named Zgemma onto the root of the memory stick and pop into the back of your Zgemma. Turn it on and the magic will happen.
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