one way to circumvent this is (if your system is already 10.5.4) to download the 10.5.5 single update from apple and place the ".pkg" file in your downloads folder (this way, at least you know its downloaded properly) then run ASU and DON'T select the 10.5.5 update... select a different update.

NOTE: The 10.5.5 update has caused quite an uproar with select people's systems and no matter how they obtain a copy of a Patched update (that works on someone else's machine) they might not get it to work on theirs.

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