El Capitan can not see an external drive

I have an iMac (27-inch, Late 2013) using El Capitan, While it will see two of my externals, it no longer sees my Seagate Backup Plus 5TB Desktop External HardDrive which has 4.3tb available. I contacted Seagate Support last week when I could see it be not write to it. They took over control and added the necessary drivers to work with El Capitan. It did work until I tried to load 1.3tb of video files onto the drive. After the process locked up, I rebooted and ran disk utility:

I attempted to move just a few small files and the drive disappeared from view and the desktop. Now, when I boot there is no indication of the drive. While running Disk Utility just now, it eventually found the drive, but "First Aid" says Operation successful as before. However the drive is not mounted on my desk top and I get "The disk you inserted was not readable by this computer"

Seagate is flummoxed. Any suggestions are gratefully accepted. Thank you.