I'm having the same problem as several folks have previously mentioned, and I don't see a solution.
I got a 4TB WD USB3 drive to back up an internal 4TB drive. The internal drive is about half full. I installed WD Backup included with the new drive, but it won't back up to this drive. It says "Unable to back up files to My Passport because there is not enough space."
What I have tried so far:
Check for updates to the WD Backup app. I have the most current version.
Reinstall the WD Backup app.
Check that the external drive is formatted NTFS. And I have since reformatted it multiple times using the WD instructions posted previously just to be sure. Still no luck, I still get the same message.
I'm also backing up my user folder to a different WD drive using the same app, and that's working just fine. It just doesn't like this 4TB drive.
Windows 10 Pro version 1607 build 17393.1198
WD Backup version 1.8.6312.2156
Windows reports free space on drive: 4,000,388,874,240 bytes / 3.63 TB
Windows reports used space on drive I'm trying to back up: 2.04 TB