The German C'T supplied a final release of Squeeze with some adaptions in accordance with the Deb-developers.
I want to get a running Squeeze onto a bootable USB-stick to test it, to make desirable adaptions and adoptions and only when I feel, that everything is okay, install that over my existing Deb-installation.
The DVD however offers only an "install" over an existing Linux-system, destroying that.
Really hours and hours of Google aso. brought no solution to a - in my opinion - simple task.
I want to transfer the content of the DVD onto the USB-stick, make that stick bootable and THEN enable me to act like the system on the stick were my real OS, including of course making changes to the configurations. Only when I'm satisfied with the result, I then want to overwrite my existing system.

Who wants to help me with this problem?

Thanks already.

Was your USB drive attached when you ran the installer? If your internal hard drive is /dev/sda then perhaps your USB drive is /dev/sdb?

That is not correct, at least not for the Debian installation media.
The installer offers very much options to install. One is: use the biggest free space, one is: use the complete drive, one is manually.
You pick the stick (as if it was a hard-drive) and install to the stick.

Pick a Debian install medium. I don't know that DVD. If it does not offer those choices it is bad.