Haven't heard of this particular issue before. However, why haven't you made a full install of the game? If you do, this problem won't matter, as you won't have to change discs at all (you just need to have Disc 2 in the drive on launch, for copy protection purposes and whatnot).
Hmmmm... I did a "maximum" install, which I imagine is a full install. (Macs have "full" install issues, so I did the standard minimum install, and then had the choice to upgrade to "recommended" or "maximum." I chose "maximum.")
However, I've done this in the past and still needed to switch discs. Maybe this is tied into the following:
ToB requires a full install. If you didn't run the recommended procedure for a Mac OS X full install of SoA, it's possible your key is incorrect. Make sure that the Alias section in your Baldur.ini is
[Alias]With ToB installed, you'll need a ToB disc (CD5) mounted to play.
My Baldur.ini file looks like this:
CD1:=Baldur's Gate II CD 1:CD1:
CD2:=Baldur's Gate II CD 2:CD2:
CD3:=Baldur's Gate II CD 3:CD3:
CD4:=Baldur's Gate II CD 4:CD4:
So maybe that's the problem- that it still asks for the prompt. I'll delete the "Baldur's Gate II" above and see what happens...
Edit: It worked! That seems to have kept the game from asking about the CDs, and I can travel all over the place. With CD 2 in the drive I went from the Docks to the Temple (which normally requires CD3), then to the Watcher's Keep, and to the Umar Hills (which requires CD4). No CDs were asked for- success! Thanks all!