TrueCrypt alternative for Mac OS X

Solution 1:

VeraCrypt

If you want operating system independence, e.g., a thumb drive that you can use on systems other than macOS, you might want to look at VeraCrypt - the successor to TrueCrypt. VeraCrypt is open source as well. Further information can be found in Encrypt FAT USB drive under El Capitan.

Solution 2:

Just use FileVault 2, which comes with El Capitan and is pretty secure.

If you need separate volumes or passwords for different projects, you can also use OS X encrypted disk images with 128 or 256 bit AES encryption. This allows you to choose to store the passphrase for each volume offline, in separate keychains or in the main keychain as you see fit.

The benefit of FileVault is whole disk hardware encryption so you should start there and then layer in encrypted disk images for projects that require even more protection or isolation.