You can save a "drive image" of the entire internal drive to a file on the external. Then restore that file if/when needed. Both sides of this process will be slow, since you're moving many, many GBs across USB.
Or you can "clone" the internal to the external. Cloning might require shrinking a partition, if the external is slightly smaller than the internal. Cloning will "waste" the entire external drive, but activating the external should be as simple as swapping hard drives.
Personally, I'd boot off a Linux LiveCD/USB and use basic Unix/Linux tools to perform the imaging, but there are smarter and friendlier utilities, too.
From a LiveCD, creating a compressed drive image could be as simple as:
Code:
sudo dd if=/dev/sda | gzip > drive.img.gz
Cloning a drive to a larger target could be as simple as:
Code:
sudo dd if=/dev/sda of=/dev/sdb
Don't just run these commands blindly, though.
