My original array is full***, so I added a mirrored pair of 2TB disks. At least I didn't have to wait through the RAID initialization; since the array was new, and empty, there was no need to duplicate the data on one to the other, unlike the first array which is RAID-5 and had to do its parity thing before it could be used. It still took six or seven hours to do the format.
***"Full" here means "only 83 gigabytes left". This is larger than any disk I had up until a few years ago; this is larger than the largest available disks just a couple years before that. On the other hand, I have 91 gigabytes of data to put there, so for practical purposes it's full.