HOWEVER, keep in mind that all drives will fail, the only thing you do not know is when.
Here's what I do: I have a primary drive that has all my photos. I have a 2nd drive that I back the first drive up to at least once a week (or just after a large shoot). I also use a Cloud service incase the house burns down. Excessive? Well, I can sleep at night. And, FWIW, 3 weeks ago my primary drive died. I got a new drive, backed the backup drive to this new primary and was back at work, didn't lose a thing. YMMV
Quote: My back up drive is full and I want to add a new back up drive to move photos from my hard drive to and external End Quote:
And what does this have to do with the LrC catalog and the images you have imported into LrC?
A Backup drive, to backup whatever, whether internal or external, has nothing to do with the drive that is holding the images you have imported into LrC.
Please Note that there are NO Images in the LrC Catalog. LrC is a Database program and only References, make a record of, your images from where they are stored. So If the drive you are saying is your Back Up drive that is full and you only have One Copy of all those image you need 2 external drives. One for copying your images to from the drive that is full and another to make a Second Copy of them as a Back Up of them.
That Full drive is Not a Backup of your images. It is the only drive your images are on.