I have the same issue. JUST received my new 4TB WD My Passport (new model) and needed to use disk utility to reformat to ExFAT. Same message. No other app is using it. Followed those instructions for installation and reformat.
UPDATE
FORGET about OS disk utility. Use WD app, “WD Drive Utilities.” Under “Drive Erase” you have all options to reformat. Worked for me!