Problem solved. I deleted the two folders (Soloists and styles) in the realtracks folder on my my ext drive. Then i made a "Symbolic Link" of the Realtracks folder on my ext. drive and put it in the BIAB folder and unchecked "custom location"in realtracks settings.
I ran the update once more and everything seems OK now.
Using "Symbolic links" is a cool way to avoid problems when upgrading, if you use custom locations of realtracks and realdrums.

Get Symbolic Linker here (works with Snow Leopard).

http://seiryu.home.comcast.net/~seiryu/symboliclinker.html