The alternative option you have available would be to make an OS backup image to store on one of the other two drives. Then for the second image you would uncheck the main drive and 100mb System Reserved boxes and select the 2tb drive to store that drive's image on the second of the other two you have installed.
This will prevent one image from being too large to store on either of the other two smaller drives. By the looks of it the separate image for the 1.5tb used up will be quite large in size and will take up quite a bit of drive space! You could be looking at filling one of the 1tb WD drives you have there!
The latest image of the 1tb host drive here ran 462gb in size with 520gb of the 920gb total in use.