It sounds like you took the logical drive you were seeing and partitioned it, then expected to see everything you copied to one partition magically appear in the other partition. That's not how this drive works. When configured for RAID 1, the drive presents itself to OS X as a single drive half its advertised capacity. OS X knows nothing about it being RAID 1 - that all hidden from OS X. I have a similar LaCie "Raid in a Box" drive. Mine is a 2big 1TB model so somewhat different that your 4big. When it's set (using the key switch on the back - see the manual) to RAID 1, it appears logically to be a single 500GB disk. You will not see two separate volumes for copy 1 and copy 2.
If the drive volume you are seeing is half the advertised size, then it is in RAID 1 configuration. If it is the full advertised size or you see multiple logical disks, then you are have it in some other configuration. Other than the indicator lights on the back, there is no way beyond that to see that it is set as RAID 1 because as I said, OS X knows nothing of it being RAID.
Now, and continuing with my 1TB drive, that logical 500GB drive can then be partitioned just like any other drive using Disk Utility. In my case, I have it partitioned 125/125/250. Each of those partitions are duplicated in the box but again, that's all hidden from OS X.
So how do you know if one the physical disks has gone bad? Assuming what you have works the same as mine, that big blue light on the front will be red or flashing red. Lights on the back will tell you which drive failed. Meanwhile, OS X knows nothing of the failure as the drive is still presenting the good drive as the same logical drive it always has. Assuming one of the drives is toast, get a replacement. I think they're even hot-swappable so when you get the replacement, you just plug it in and let the software in the drive box take care of doing the needed copying.
