Not sure which program you are using to put the Snow Leopard image on your USB stick, but I can recommend TransMac, as it is then virtually a one-step process from the DVD straight to your USB stick, as follows:-
Install TransMac and then run the program. Insert the DVD into your drive and the larger USB drive into an available port. Right-click on your DVD, select "Save Image of Disk" and save it to a folder on your hard drive. Wait for this to finish. Next, right-click on your flash drive, highlight "Format Disk" and select "Format with Disk Image". Select the DVD image you just created, "Open", and then click "Yes". Wait for this to finish, this will probably take at least half an hour.
You might already have tried this, but it worked for me! (Using Mechdrew's No. 2 method.)
I also used TransMac to transfer the NBI image.