Problems with "Sector-by-Sector" Backup

Dear Community,

I want to perform an sector-by-sector Backup of a Notebooks Harddrive (500GB).
I am using an 2TB (empty, NTFS) external Harddrive connected via e-sata.
Acronis keeps telling me, that the Harddrive is full after about 90%, what am I doing wrong? Or is the Image really that big?

Thanks for your help

Sascha