Easystore "not enough room" Notification

I am a video editor. I have four internal drives. I have a 4TB WD Passport set up for backup. Setting up the Passport was fine; hourly backups proceeded fine until a couple days ago, I discovered that the drive had not been backing up since late February. On that drive there was 3.63TB available and 3.70GB free. That’s around 10%. (by the way, I was never informed that the Passport had ceased backing up.) I presumed that Win10 needed more space to work. I am not now shooting, my video files are on my D drive. I am not now shooting so I decided to nuke the D drive on my Passport and to back up it up on another external. I bought an external 1TB easystore at bestbuy. There is 613 GBs on the D drive. There is 931 Gbs available on easystore. When I attempt to back up I get this error message. “Unable to back up files because there is not enough room.”

What is going on?

I have reformatted easystore and uninstalled and reinstalled WD Backup. Frustrating!!

Copy and pasting seems to work.

Do you rename or modify the backup folder? If so, then WD Backup is starting over and making duplicates of your system, which will quickly fill-up your drive. This will also happen if you keep adding new large files into your system since they will not be deleted from the drive.

I have not renamed or modified the backup folder.