I've already done my 'work' and 'backup' RAID sets, the last and the largest was the 'data' set. In Disk Utility, this is what it looked like. Two 2Tb disks.
Note: At this point, I did a full back up of the data, just in case something went wrong. I'd advise the same if you are going to try this.
With the RAID Set selected, I clicked one of the disks in the array. This selected the RAID slice for me.
Then, I clicked Delete and confirmed the option.
After a short pause, Disk Utility refreshed, the RAID Set was no longer there; the two individual disks were present as expected. Since this was a mirrored set, each of the volumes is an identical copy.
Then, I shut down, removed the 'spare' disk and booted up again, Disk Utility is showing just the one disk as expected.
A quick check of the disk showed no errors, success!