atacontrol and creating raid-1 arrays

Hi,

I have just done a 'cp /dev/ad4 /dev/ad6'. Those drives should be 100% 
identical (even the same drive models). Will the command
'atacontrol create RAID1 ad4 ad6' ruin the data on my drives?

I ask because I am hoping to avoid the need for another 150 gigs of 
space just to copy the data off of the drives before creating the 
array...

Anyone?

/Eirik