In case anyone stumbles on this thread, I didn't want to leave it open even though Dell never responded to my latest questions.  I did switch out the USB stick to a 2.0 version stick and it worked.  I'm not sure why internal USB is built to different specs than the external USB's.
 Hi, I have a brand new Samsung 32GB Fit flash drive in the internal USB slot.  I installed ESXi 6.7 to it.  Upon rebooting it seems to get stuck at "Initializing Firmware Interfaces".  I let it sit there for about 10 minutes while I researched what could be the issue.  One site mentioned removing the USB stick.  Did that, and it literally went on to the next step as I pulled the flash drive out.  So I threw it in the front USB slot.  ESXi loaded just fine.  Is there something special I need to do to get the internal USB to function like the other USB slots? Thanks for any advice!
Dell R720XD Stuck @ Initializing f/w when using internal USB