Enlarging Acronis Secure Zone doesn`t work

Hello,

The programm refuses to make my secure zone bigger than 50 GB
although my harddisk (1 TB) has left about 800 GB free space.

For saving may System&Programms a minimum of 80 GB is needed.
I tried to enlarge to 100 GB, but the size is limited to about 50 GB.
I cannot make Backups into SZ any more :-(

I defragmented my disk...without any result.

I use Acronis True Image Home 2010 on Win7 64bit.

Can someone help?