You did not specify what kind of Linux you are using.
Also not whether your disks are SATA/PATA etc.
But generally, you can try the dd command.
First, open a terminal, and execute:
dmesg | grep sd
exactly like that, it should give you a list of your harddrives (/dev/sda or /dev/sdb). Once you know which is which, by looking at the size of the drives, you can try the dd command below:
sudo dd if=/dev/sda of=/dev/sdb bs=1M
NOTE: In this example /dev/sda is the source/read device and /dev/sdb is the target/destination device, adjust accordingly. This command will read from disk /dev/sda and write to /dev/sdb, causing /dev/sdb to be an exact copy of /dev/sda. /dev/sdb needs to be at least as large as /dev/sda.
WARNING: any mistakes with the dd-command, such as using the wrong device names, may lead to total data loss. Use with care!
...and just as a cautionary note - don't type this command in verbatim! As sub mesa described, make sure you know which device ("/dev/sdx") is which before you start and use the right device names in the right positions in the command. That last thing you want to do is copy in the wrong direction!