Encrypting Data on part of a file system.

Question

A few months ago, using hints I found on the internet, I was able to use diskutil command line utililty to create an encrypted partition of the same sort as when turning FileVault on in Security Preferences.  File Vault doe not appear to offer a way to choose some pargt of the disk storage such as an entire drive of a folder on a drive.  I was able to do it and it worked.  When I mount the disk partition to the system (usualy by plugging it in and turning it on), I'm asked for the security pass phrase or key to decrypt it.  Once mounted with the key supplied, I can access it as any other mounted disk with the type of access restrictions that might be present on any disk.Since I want the data to be truly privatem U decline to put the key into the a known place such as the keychain.  I don't want just anyone who has a log on to this iMac to b e able to read this data.  I want them to need to enter a private key to mount the data. 
My only problem with this is the hoops I needed to go through to do this.  It is complicated and invovlves setting up special partitions for the purpose.
Searching Finder help for encrypting data it offered a solution for data on a removable drive.  The stepsare very simple and easy to do:
   a) Mount the files to be encrypted if they are not  online.  They also need to be in a folder or even an entire partition.
    b) Open Disk Utility (GUI version)
    c)Choose File > New > Disk Image From Folder (or New-> Disk Image ffrom a Device).
    d) Select the folder or disk you want to encrypt.
    e) A save dialog will pop up.  Select the name of the archive you wish to create and select a location.  I choose a removable disk partition which has enouh space.  Select Compressed if you wish.  Then Select Encryption and choose the key size for encryption from the drop dwon.  When you click Save, Disk Utility begins creating a disk image that is (possibly) compressed and probably encrypted.  Once done, the files in the folder or partiion are hiddent behind the encryption.  To get to them, you much open the DMG file and supply the password to unlock the encryption.  You can save the key in the keychain if you are not worreid about who can get in.  If you wish to restrict access to fewer people, keep the key secret and provide a recovery mechanism that is suitable for you need.
   f)  One the archive is created, the disk partition containing it may b4 mounted on the system (if it is not there already) and by opening the dmg file you will be asked for the key.  The system will validate that the key works and the encryption and comprewssion are working.  The archive will be mounted as a virual disk.  It can be accessed by any useer of that computer unless the file permissions get in the way.  Mounting it only when the computer is being used by authorized people allow you to mount and dismount the archive for use during a limited time.
I have a couple of questions here.  Is there an easier way to do this?  Is this encryption as strong as that used in FileVault? 

Answer 1

No. I don't know why it would not be, except it is easier for a person to leave the disk mounted where anyone can then see it. With FileVault forcing a password on wake from sleep, it will likely be encrypted if anyone found it.
I'm not sure why you went to the trouble you did before, except the instructions might have been to create an encrypted partition as opposed to creating the disk image. Disk images have been around for at least a decade.
If you plan on backing up the image with Time Machine, use a sparse bundle disk image as it will write the data to small files, called stripes. Only the stripes that change get backed up instead of the entire image.