I was unable to find how to rebuild the RAID from disk utility after a couple of hours of research. However, I managed to do it from the command line.
sudo diskutil list // to get a list of disks. Note the disk number of your new disk (e.g. disk4)
sudo diskutil appleraid list // note the uuid of the raid in question
sudo diskutil appleraid add member disk4 UUID-OF-YOUR-RAID
It will then kick you back to disk utility and start rebuilding the raid :)
Also, if the failed disk remains on the list, you can remove it:
diskutil appleraid remove UUID-OF-FAILED-DISK UUID-OF-RAID
Firstly, RAID shouldn't include your backups. Something catastrophic hitting one, will hit the other. Eg. fire, burglary, etc.
Ideally, you want to keep one set off site. Using the 2 disks you can keep one at a friends, and sync the up periodically. There are tools that allow you to do this. Alternatively, something like BackBlaze, Amazon Glacier, will give you huge or unlimited storage that is remote and protected.
Secondly, when setting up RAID you want to think about your priorities. Are you optimizing for speed, redundancy, etc.
I you want to have one unit in house, and one remote and periodically updated, you're probably better off with RAID5, which is a better mixture of redundancy and speed.
Finally, RAID won't allow you to upgrade disks singly.
Alternatively, using a something like Zevo's ZFS you can configure the Big5 as a JBOD and feed the disks into a ZFS RAIDZ. This will allow you to mix and match disk sizes, and upgrade over time. It will also keep better control of your disks, checksum all the data, and a host of other features.
Personally, I'm a big fan of the ZFS approach, as I think the performance and security are better than a plain RAID.
RAIDing using OSX software, on top of a RAID in the LaCie box is asking for some complex failure.
Best Answer
I think what is going on here is both expected behavior and an exercise in learning a little terminology.
It is normal for a Mac to see a third party created RAID array as a single disk (which I am assuming it does) In both the Finder and Disk Utility? Why? Because the LaCie utility is designed to create RAID arrays that are completely transparent (or if you prefer invisible) to the operating system.
The converse is also correct, if you create a RAID array with Apple Disk Utility the LaCie utility will just see it as a single disk.
I work with Windows/ESX servers at work and RAID is created in the firmware of the RAID controller, before the operating system even loads. So both Windows and ESX see the RAID slices as individual disks even though the RAID array knows that there are 6 physical drives set up in a RAID-5 array.
There may be reasons to use the LaCie software over Apple's Disk Utility (or vice-versa) but I am not aware of the relative merits of each utility.
BUT You can be assured that what you are seeing (if I understand correctly) is entirely normal and expected behavior.