That was the premise of my original post... for some operations, Flex or the Drum Replacement feature must move the audio data into RAM, or at least a copy of the audio data. Flex is done in real time so it seems that Logic would be juggling considerable amounts of data when using multiple flex time tracks.As mentioned before...
However, when I reduced the project to only 8 flex drum tracks, I was able to use the Drum Replacement feature. But then why would eliminating 24 audio tracks + 3 flex tracks make the difference? Would audio tracks use RAM?!?
Maybe it was mostly because of the operation itself on a 5 minute drum track... That would confirm that some operations would use a lot of RAM.