Hard drive encryption is an integral step for anyone looking to secure their data. It is a process which will go a step beyond regular password security and will ensure that a hard drive remain secure only to the user. However, it is important to note that once a hard drive is encrypted, ITS can not remove the encryption so any files that cannot be accessed may remain that way until the user themselves can access them.
There are both paid and free software for OSX and WIndows, this guide will focus mainly on OSX 10.6 and above and Windows 7 and above.
Note: Drive encryption may require you to format your drive.
Apple Disk Encryption (File Vault)
Microsoft Windows BitLocker (free)
Alternative Encrypting Applications.
If the default operating system encryption methods don't work for you or you want something a little more secure or efficient, there are a few other third-party applications out there for your convenience. Some recommendations include:
GNU Privacy Guard (WIndows, OSX, Linux) - Free
TrueCrypt (Windows, OSX, Linux) - Free
7-Zip (Windows) - Free
AxCrypt (Windows) - Free