Hi,
I was able to solve that issue!
As clonezilla didn't work I tested several other tools, and Ghost4Linux on current Parted Magic CD worked fine! I simply did a raw copy on another harddisk, and was able to boot from that one. Its an old Seagate ST51080N, which is way quiter, and has 1GB (there's also an image on our network now ... just in case we'll need it in the future).
Thank you very much for your time & hints!
cu
Torsten