Thank you so much for responding. I did install the WD apps, then I deleted them because my startup disk was full.
I also deleted a couple PDF files and I think this freed up enough space-- because when I tried formating again/partition, it worked!! But, I forgot to reinstall apps.
So, it seems the problem was one of space.
I know the users manual is online-- but do you know if the apps are available too?
How much free space is on the HD, where has all the space gone?
OmniDiskSweeper is likely the easiest/best, and is now free...
And if needed, you can use Terminal to run it in admin mode to see files/folders your user can't see...