Advice Plz... HD Videos to PS3 with ReadyNAS Ultra 2

Yes it is. When you replace a drive with another of equal or larger size, it will resync the drives, in your case, synchronizing the data sector by sector on the old drive with the new drive. Then when you replace the second drive with a larger drive, it will again synchronize the data on the other drive to the second drive. It will probably require a reboot after the resync on the second drive to expand vertically. (Horizontal expansion is achieved by adding drives, vertical expansion is achieved by adding larger drives. The vertical expansion will not take place until there is redundancy - at least two drives in a unit with single redundancy).