I'm using truecrypt to encrypt hdd partitions and files since last 6 years. The interface may seem a little complicated, but once you get used to it, it is very easy to configure. You can use truecrypt to encrypt the full hdd, or a partition. You can also create encrypted file containers and use them just like any file. I will mention the basic steps here for you.
Open truecrypt, press the button 'Create Volume' to open the volume creation wizard. Select 'Encrypt a non-system partition/drive' > Next > 'Standard Truecrypt Volume' > Next > 'Select Device'. Here you select the external hdd > Next > 'Create encrypted volume and format it' if there is no data in .the disk. If there is data already then select the option 'Encrypt partition in place'. First option is recommended, you can later copy the files. > Next > Encryption algorithm. I use the default 'AES'. > Next > Next > Enter your password. Minimum 20 characters is recommended, but you can use even a single character. Optionally you can also use a 'keyfile' like a picture or audio or video file which will be required later to decrypt the container. > Next > 'Yes' for Large Files > Next > Format. This will format the drive and encrypt it. After the format is complete, close the wizard. There is no finish button, if you press the next button, the wizard will start from the beginning.
In the main window you can see partition letters, click any one of them, click 'Select Device', then click 'Mount'. you will be prompted to enter the password and the drive will mount as a normal partition in my computer. You can also add a few options in 'settings' menu like 'open explorer window for successfully mounted volumes' and 'auto dismount volume if no data is read/written after the specified time.
Even if you forget to dismount the drive, the partition will dismounted automatically on reboot.
You can even create a batch file for automatic mounting at boot time if you wish.
