Problem backing up with EaseUS Todo Backup Ree 11.5

[Broadback]

I have backed up and restored using this program successfully a number of times. I use 2 Toshiba 1 TB  disks alternately. Now when trying to back up I get an error message of insufficient space after some run time. As there is 930Gb  free space on the target disc and the source disc is only 396Gb I am at a loss. I have tried reloading the program from scratch and full format of the target disc (yawn) to no avail. any suggestions please? I do not wish to try the alternative disc as that would perhaps corrupt my only backup!   

[Simon]

If you've completely formatted the drive successfully, and it is still reporting low space, then I'm wondering if there is a problem with the drive itself?

What does the drive look like in My Computer?  Is the error message coming from Windows or from the backup utility itself?

[Broadback]

From the backup utility. there are no errors that I can see anywhere.

[Glenn]

Has the drive been partitioned?

What does it show in Disk Manager?

[Broadback]

Disk manager shows it as a healthy (primary) partition. I have copied the backup from the other drivre to this one, it copied fine, however, not surprisingly it will not rim with EaseUs. I tried another backup with the same failure.

[Broadback]

It has, all on its lonesome, started working!