Just found out that Acronis True Image 2010 to build 7046 gives an error message when you try to run a backup.
"Target Drive is running out of Space".
You need to go to
Tools & Utilities
Local Storage Setting
You will probably find it is set to warn you if there is less than a staggering 50TB of space. (50,000 Gbs) Change this to 50Gb or better still select the default option so as to not warn you at all.
Even if you select an incremental backup it will create a temporary full backup as well.