Some more background: When connected to a SATA connector, in Linux drives are called /dev/sdx, where x is a letter a,b,c,... First drive is a, second b, ...
Your two drives are probably called /dev/sdb and /dev/sdc, assuming the 'default disk' is /dev/sda.
you can find the drivenames by typing (yes indeed, in a terminal)
sudo fdisk -l
This will list the drives, with their partition tables. The old LaCie disk has about 5 partitions, the 2nd being the biggest. The new drive will be empty, I suppose. The remaining drive(s) will be the system drive of your computer. The sudo (superuser do) command is necessary because a standard user doesn't have the rights to execute fdisk.
When you have located the drives, you can do the copying:
sudo dd if=/dev/sdb of=/dev/sdc
Doublecheck that if (inputfile) and of (outputfile) are the right drives! The data on of will be lost forever.
This operation will take several hours (250GB in about 50MB/sec), and there is no feedback until it's finished. (Besides the disk sounds)
After this is done, the Lacie should be able to boot from the new disk.