Which is faster: Encrypt empty external disk or leave data on disk while encrypting I'm trying to encrypt one of my 2 TB external drives using Finder > Right-click on drive name > "Encrypt"

I moved all the files (~1 TB) to another drive because I was thinking that an empty drive will be encrypted faster than encrypting the drive with the files in place. 

However, I'm about four hours into the encryption process of the empty drive and it's only about 5% done. 

Is there any advantage to encrypting the drive while it's empty, or does having files on the drive not make a difference in encryption speed? 

This is on 10.12.3.
Leave the data on the disk being encrypted to achieve the fastest speed.

Full-disk encryption encrypts the entire disk, no matter what data is or isn’t there. Otherwise an adversary could easily determine which sections were blank, and attack only the ones that weren’t all 0s.

That’s one weakness of per-file encryption: allowing an attacker to determine what they want to target.

The way you chose to perform this operation, your empty disk will be fully encrypted once. Then once you move the 1 TB back onto it, encryption will be performed a second time on half of your drive.

This means it will have taken 50% longer to encrypt the drive vs. if you had left the files on the drive to begin with. Then add to that the amount of time required to shuffle the data back and forth, and you end up with a significant time loss.

Now since the process has already started, it’s more advantageous to let it complete before moving the files back over. Spinning hard drives really struggle with writing multiple areas of a drive at a time (which would occur if you encrypt one area while copying files to another).

If the external is an SSD, copying while the initial encryption takes place might not impact the process as much.