Hmm, I don't know if they come preformatted or not LadyC. I haven't got any external hard drives myself. They will need to be formatted in NTFS file format at some point.
I keep my ISO's on one of my internal drives, but I also make backup disks. Since I don't want to spend tremendous amounts for 50gb blank disks, I use the much cheaper 25gb ones. I've paid $5 apiece for BD-RE (rewriteable) disks off ebay, and recently bought some BD-R disks for $1.80 each from Newegg when they went on sale, so it is possible to find good deals on BD blank media but it takes a lot of work to hunt them down sometimes.
Many Blu ray movies won't fit onto 25gb disks without either taking out extras, or compressing the movie. These require a little work to fit onto the 25gb disks, but can be done. If they need compression, you can use a free program called BD-Rebuilder. You can make movie only or complete backups with it.
If you only want the main movie and it will fit without compression, you can use ClownBD. It is quite handy for those movies with seamless branching where the main movie is split among several M2TS files.
BD-Rebuilder can compress enough to fit the movies to regular dvds also, either single or double layer. These are still in Blu ray format, so they won't play on a stand alone dvd player, and may not work in your particular Blu ray player either. Some Blu ray players are just picky about that.
BD-Rebuilder requires a very powerful computer and lots of time for encoding, in case you ever want to play around with it. It doesn't include a decrypter, so you have to use AnyDVD HD to rip to the hard drive first.
To burn Blu ray movies you can use ImgBurn.
Hope that gives you some useful information about backing up Blu ray movies. Let me know if you have any more questions on it.