Thanks for the replies.  We found out it is a problem with the anti-virus program blocking the USB drive.  Once the anti-virus software is uninstalled, the external USB drives are recognized.  The software company is working on a fix and recommended rebooting before using the USB drive, or uninstalling the anti-virus program until the bug is fixed.  Thanks for your replies.
I ran into a problem this week with 3 Dell computers with Windows 10--an Inspiron desktop, Optiplex desktop and Inspiron laptop.  The USB flash drives only work after once after an initial reboot of the Windows 10 computers.  Once unplugged and plugged back in, the USB drives are not recognized until rebooting the computers.  I have tried several flash drives and different ports on each of the computers with the same results.  The three computers have all Windows 10 and Dell updates installed, including the BIOS and chipset updates.  I  have done a refresh of the USB ports.  Nothing in the Device Manager is showing as Unknown. Anyone else run into this?  Any suggestions for solutions are greatly appreciated.  Thanks! B Stevenson    
Issue with USB Drives--Only Works One Time after Reboot