In this case you have to stop using your external hard drive and do not add any new data on it and then use suitable photo recovery software. You can find lots of photo recovery software online. But among them I would recommend you to use Digital Media recovery tool that is designed to recover media files like photos, videos and audio files from hard drive, USB flash drive, external hard drive, memory cards and other data storage devices.
You can download the software from here – http://www.rescuedigitalmedia.com/photo-recovery-from-hard-disk-drives-on-macwindows
This software offer free trial version in which you can scan and preview the recoverable files within the software and if you are able to preview your lost or missing photos you can then proceed further to purchase the full version and easily recover photos.
May my suggestion could help you.