First, I suggest you do not use the on board RAID facilities. The reason for that is that your RAID depends on the chipset on the motherboard, if your motherboard fails, you could end up having a hard time gaining access back to your data.
I always recommend setting up RAID using a separate card. This is the card I use for my clients
It is very inexpensive, I keep them in stock in case one fails. When a client wants to upgrade to a new computer, I simply take the card and the two RAIDed drives to the new machine, boot into safe mode, install drivers for the new machine and in a matter of 20 minutes have everything running just as if it were running on the old machine. The only way you could do that as smoothly if you are using the motherboard RAID is if the new board happened to use the same chipset (which is often not the case.)
Also, the Gigabyte RAID is actually a JMicron chip. JMicron chips are truly mediocre. A lot of manufacturers use them because they are cheap and, that's what they really are cheap. I was trying to be kind but, they are not just cheap, they are junk.
The intel stuff is really good but makes access to your data dependent on the motherboard's chipset. In case of a motherboard problem, it makes for a bad situation.
The card I suggested uses a Silicon Image sil3132. A very good little chip. The only thing I don't like about it is that their RAIDmanager utility uses Java and I really dislike Java. I have not found a "perfect" solution that can be had for a reasonable cost. The closest is the card I suggested. I've installed plenty of them and can vouch that they work quite well and their performance is also very good.
If you follow my suggestion, that card (as well as Intel's stuff and the Gigabyte RAID) ask you which drive has the data that must be copied to the other drive. Just make sure you don't tell it to copy the empty drive onto the drive that has data. As long as you pick the correct drive to copy from, you won't lose anything at all.
By the way, I derive no benefit whatsoever from recommending the RAID card I mentioned above.
Well, I'm quite happy with Intel's RAID.
But when I try to create a RAID array, it tells me that all the data will be lost... I don't want to erase the data... I just want it on RAID... can't it just copy, as you said?
Every RAID controller I've used has always asked for a source and target drive for data, maybe the intel one does not offer that option. As you know, I stay away from the motherboard implementations of RAID.
If it doesn't give you the option to copy then the only thing I can think of is for you to make a copy of your drive onto yet another drive (thereby saving the data), create the array and then copy the data onto the array from the other drive.
Hopefully someone that is better versed with the intel RAID implementation can offer a better solution.