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. 

1. Apple OSX 10.8 Disk Encryption (free) 

OSX 10.8 (Mountain Lion) provides an easy and very simple way for users to encrypt their external drives. 

...

To remove the encryption simply right click on the drive and select Decrypt. 

2.Microsoft Windows BitLocker (free)

BitLocker is Windows' built in hard drive encryption software. It works on Windows Vista, 7, and 8. This guide will be using Windows 8 but the directions are mostly the same on all 3 operating systems.

...

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

...