UBUNTU18.4 freezes when I try to mount ext HD

Hi, I am a beginner Ubuntu user. I am running Ubuntu 18.4 Everything was running fine until I was deleting some files from a 4T ext HD running on a SATA3 box. Ubuntu froze, I rebooted the PC and everything works fine until I try to access the drive. When I do everything freezes. Used DISKS to look at the drive and it shows UNMOUNTED. If I click on MOUNT, UBUNTU freezes. I checked the HD with SMARTS and it reports it as "Disk is OK" There are two other HDs in the SATA box and they work fine. I can't seem to mount the drive. This was my MEDIA drive for PLEX. I am not sure how to remount the drive via terminal, but if I try to CD to the drive I can,t. If I click on the drive it will list all the directories, but then it immediately freezes.
Any suggestions, or what further info I should provide to better diagnose the problem.?????
P.S. I just realized that my Ubuntu desktop background is no longer the default, its black. I did not change it. Significant?
I JUST discovered that if I click on the drive and the desktop freezes, I can STILL access the media files in the HD on my TV via the PLEX client in my ROKU. Which tells me the HD is OK and its the desktop screwing up. Still need help to fix this. Thank you guys for any help you can provide.