Its not a question but here is what i found out how to setup pc in order to backup systempartition with true-image-boot-cd.
tried to start pc with external drive also ready and at the same time running boot-cd. didnt work.
the solution:
1. startup pc
2. start external esata-drive until recognized.
3. restart pc and boot to optical drive where boot-cd is located.
Once was so desperate that instead of waiting i intervened too early mixing up partition table when chdk was running resulting in overlapping partitions on a backup-drive mounted an enclosure. still unsolved.