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 macOS and Windows and they are listed below.
Note: Drive encryption may require you to format your drive.
Microsoft Windows BitLocker
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:
Link to GNU Privacy Guard web page (Windows, OSX, Linux) - Free
Link to TrueCrypt web page (Windows, OSX, Linux) - Free
Link to 7-Zip web page (Windows) - Free
Link to AxCrypt web site (Windows) - Free