Install a second hard disk from a first one (disk cloning)
Don´t copy partitions (if you have more than one) because then you still have the same partition sizes. Instead make the partitons on the new disk bigger and then copy all files from the old disk to the new disk. And if you copy the partitions it is no yet bootable. You should run the command grub-install with the appropriate arguments to install grub to the boot sector of the new disk. And you will have to adjust /boot/grub/menu.lst and /etc/fstab of the new disk to use the correct UUIDs for the partitions of the new disk. More on disk cloning, Upgrade a Hard Disk and Hard Disk Upgrade Mini How-To.