VISA file write works on 32 bit WIN 7 but not 64 bit

I've got an application that has worked for years on several XP machines. It works on WIN 7 32 bit as well. It does NOT work on WIN 7 64 bit.

I download a file as a binary, break it into 4096 bit packets and send each packet via VISA write to a UUT. I've tried setting the program to run in XP mode, run as administrator. I suspect it's the VISA drivers, and I've downloaded and installed NI-DAQ 9.8 but I'm not having any luck