Actually I have traced the error to sendfile.cpp on line #136. Seekg function in filestream returns - value when trying to find the end position in the stream. When hardcoding the 3 gig size value in size, send file executes fine. Need to resolve this issue for seekg not being able to determine the end of file in large file sizes. Not an issue with 1.5 gig files (tested). Only seems to be an issue over 3 gigs.