There is no need to download SP1a if your computer already has SP1. SP1a is simply SP1 without Microsoft's Java Virtual Machine. Microsoft removed VM to comply with the court's ruling against them. Microsoft also released SP1b (SP1 without VM, and with Sun's Java) to comply with the court's other demands.
Drives I Own:
Asus S520/A, Plextor UltraPlex 40TSi, Lite-on XJ-HD166S, Pioneer DVD-106S, Toshiba SD-M1712, Plextor PX-R820Ti, Plextor PX-W1210TS, Plextor Premium, Samsung SM-352B, Teac CD-W540E, Yamaha CRW-F1, Plextor PX-708A, Sony DRU-510A