I had a similar experience with my S500 where I had 250GB and attempted to add 500GB to the array aggr0 and they would not join the aggr. I opened up a case and they took 48 hours only to say "sorry you cannot mix drives". Good luck but I would open a case with support before you make a move and have them confim this - trust but verify.
I was also told that it is not currently supported for the S500 for mixing drive sizes. However, I raised the question over on the old Storevaulthelp.com forums about this since the 250GB's were going away. If my memory serves me, I was told that this feature will be available in an OnTap update in the future.