Actualy, encryption the entire hard disk will stop anyone from acessing the drive without authorization. Software encryption is possible, it tends to eat up system resources and slow the machine down considerably. It also tends to use a weaker encryption to speed things up, thus its not as secure as it could be. To be totaly secure, you need a hardware encryption module.
Such a device usualy takes the form of a card which plugs into an IDE, SIDE, or SCSI port and has an identical interface on the oposite side. The drive cable is then plugged into the card. All information going into the drive is passed through the encryption system on the card, shifting the actual bits around as they are written to the disk. The disk, if removed from the card, is useless because all of its contents are encrypted, even the file system.
To acess the drive, you must use an encryption card identical to the one the disk was written to with and have the same security key used to write to the disk. The security key can take many forms, usualy a USB pen drive with a large "password" which is the key to the encrypition algorythim. The security key may also use a propriatary connection, other than USB.
The only way to bypass this system is to employ brute force. This means using trial and error to attempt to find the key. This will usualy take a very long time because keys tend to be hundreds of bytes.