RAID 1 with Two LaCie 5Big Thunderbolt I'm looking at getting a LaCie 5Big Thunderbolt unit for archiving purposes. I would like to get a second one and use Disk Utility to create a Mirrored RAID Set from both of them so that I have a backup of my data. Both LaCie units would be set to be RAID 0 so that I have maximum space available. Am I able to do what I intend to do?

My next question is, assuming I can set them up as a Mirrored RAID Set, would I then be able to swap out a drive on each of them (one at a time, giving the RAID time to rebuild) to increase storage capabilities?

I'm currently planning on getting two 10TB units as that will supply me with enough storage for quite some time. However, I would like to be able to swap out the smaller drives with larger ones in the future.
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.