Ho do I make a drive the system Drive?

[gabiwabi]

Hi all,
I have run out of disk space on my Mac Pro.
I have the drive where the OS is installed (250GB) and another drive that is 400GB.
How do I "copy/cloan" the original drive to the 400GB,and make the 400GB drive my main bootable drive.

Many thanks,

Gabi

[dnanian]

There are many posts about this exact topic here in the forums, Gabi, even some from the past day or two, so take a little browse around and I think you'll find your answer quite readily! :)