That sounds like a system failure to me, but that's a guess. If it were me, I would start an online chat with Adobe where they can troubleshoot your system. Select Online Support in the help menu. That takes you to a website. In the lower right is an icon that will start a chat session.
I had a simiar problem. It turned out i had my external HDs fomrated exfat so I could take and plug them into a PC. I could add media from my internal hard drive because it was mac formated. So I copied my external HDs over to another drive, reformated them to Mac, copied my date back to the newly mac formatted external HDs Everything works great now. I suggest look at your Eternal Hard Drive format and see what it is. Good luck.