Could you please reset the Preferences of Lightroom once.
How to Reset Preferences : Resetting (or 'Trashing') the Lightroom Preferences file - Lightroom Forums
Let me know if it helps.
Thanks for the advice. Regrettably, the problem remains.
could you please check and make sure Lightroom is updated to the latest version, that is Lightroom 5.7.1
It is the latest version. I have discovered that I can print from internal HD but NOT from external HD.
Please make sure that you have Read and Write Permissions on the External Drive.