Yeah , this is because you are using mixed sized disk's in an array essentially you will be limited to 2 X 500 gig , the array controller will be treating the bigger platter as a 500g disk and ignoring the extra bit . So your capactity will be around a TB of fast usable storage which is what your getting its not really smart to mix different sized disks for support reasons ... although i have done some horrifc step ups before where ive hot swapped disks to larger disks in raid 1 sets to grow storage on running DB clusters in my time.