TrueCrypt is an open-source encryption software that enables you to create a virtual encrypted disk within a file and mount it as a virtual drive, that can be accessed via a drive letter.

Any file that is stored on this virtual drive is automatically encrypted on-the-fly, and can only be accessed while the drive is mounted and unlocked with the correct password or key.

TrueCrypt supports a variety of encryption algorithms, including AES-256, Blowfish (448-bit key), CAST5, Serpent, Triple DES, and Twofish. Other features include full disk encryption, support for FAT32 or NTFS formatting, hidden volumes, hotkeys for mounting/dismounting and more.

TrueCrypt can be installed on a portable drive unless you want to encrypt your system partitions.

Note:The development of TrueCrypt was officially ended in May 2014, based on (controversial) concerns over potential security flaws. However, a crowdfunded audit of the TrueCrypt source in 2015 has not found any flaws that would make TruCrypt insecure.