The ES.3 come in SAS or SATA, so it would have to be the SAS version. You could add a 1TB SAS disk to the existing RAID 5, HOWEVER, only 300GB of it will be usable in the array, and the remainder will not be usable at all (unless at some future point you plan to replace all existing drives with 1TB drives).  Otherwise, you could simply expand using another 300GB disk: [URL]   I'm not sure how much/little you were expecting, but RAID 5 always uses ONE disk's worth of space for parity, whether you have three disks in RAID 5 or ten disks, so 3x300GB in RAID 5 = 600GB, and 4x300GB in RAID 5 = 900GB.
Our 2950 has SAS 300 Gb 15k drives in it. It is full so we need larger drives. I will have to do the one at a time slow and wait method, no place to back it up to. Do the new drives have to be 15k? We need to buy cheaper drives. thanks Lee
Dell Poweredge 2950 new hard drives