Is a drive with El Capitan installed likely encrypted?

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Can I take the drive out of a Mac OS El Capitan computer, connect it to a computer with abilities to read HFS+ (via USB), and read all of the user data?

From what I read, El Capitan installations could be using FireVault. (From what I understand, FireVault started with Mac OS Yosemite.)

If it turns out that the drive is encrypted, my next idea would be to turn off the encryption (by booting the drive normally), though I am guessing that not possible to turn off the encryption.

Maybe the drive becomes completely useless once removed from the computer (except to reformat)?

 

[!!!]

From what I understand, FireVault started with Mac OS Yosemite.)

FileVault FDE started with Lion.

my next idea would be to turn off the encryption (by booting the drive normally), though I am guessing that not possible to turn off the encryption.

You could do that, as long as you can log into an admin account.

Maybe the drive becomes completely useless once removed from the computer (except to reformat)?

No, you could unlock the drive on the other computer if you have the password.