I have a system that is already installed and I was wondering if I could encrypt the HDD with out reloading the os. I know luks is what rhel 6 uses but I do not know how to do that with out reloading the os.

Any help would be great.

Thanks

As far as I know a partition of a living os cannot be altered (encrypted).

Indeed. And if you do feel the need to encrypt a volume, save yourself a lot of work and get a SAN or similar device which will do the encryption for you in the disk-controller hardware.

Ideally, one needs to have started by installing a distro that asks you if you want encryption during the installation! But you can encrypt /home later with a bit of hard work. I've never tried it, but the information is (as ever) out there:
http://robert.penz.name/145/home_par...s_under_linux/
https://www.cs.arizona.edu/computing...ncryption.html

what if I boot off of a disk and then encrypt the disk? Will that work or will any way I encrypt a disk reformat the disk?

You can do a full disk encryption (all but a small boot aprtition) from most distro isntall disks. Just do your reading and planning ahead of time. It is not possible to encrypt a drive in situ in Linux.

... and may I repeat ... it's quite inefficient (IMHO) to do this sort of thing in software.

What you're really trying to guard against, I think, is the possibility that the disk is stolen, or unscrupulously duplicated. A disk controller, or in some cases the drive itself, can have hardware encryption facilities which encrypt, decrypt, and validate the data with no loss of speed. Many government contracts (including non-classified ones) mandate the use of such facilities, so the technology is out there. To the properly-authorized computer, the drive is "perfectly ordinary." But if you steal it, it's useless to you. Furthermore, if you attempt to use the wrong key, it knows you're trying to use the wrong key, and it won't let you do anything ... but of course you can't deduce what 'the right key' is.

(Alice and Bob can use their data as they ordinarily would, and as quickly, without suffering inconvenience, while Eve says, "curses! foiled again!" Exactly as good encryption ought to be ...)

Actually, recent versions of cryptsetup (currently 1.6.0) include an optional re-encryption tool that is capable of adding LUKS encryption to an existing partition. But, you really need to know what you are doing (e.g., shrinking the filesystem to make space for the LUKS header), and have a current backup. The (lengthy) process can be suspended and resumed, but there are many opportunities for unrecoverable errors, especially when the filesystem is being shifted to make space for the LUKS header.