I was under the impression they never released any boxes designated as SAT-530. This was because they dropped the Zenith name from the STB line and went to LG designations (Zenith is LG). The HD-SAT530 name was replaced with the LG 3200A.
The HD-SAT520 has a flaw in that it's memory gets erased on power failure - so you loose all custom menus and favorites (It is the same box as the Sony HD200). The LG3200A (aka HD-SAT530) fixed this problem and with the latest firmware is considered by many to be the best DirecTV HD box (along with its clones the Sony HD300 and Hughes HTL-HD).