You can use good old `dd` for this:

    dd if=/dev/zero of=test bs=1k seek=2m count=1

Or python:

    python -c 'f=open("test", "w"); f.seek(2e9); f.write("\x00")'

Both these calls create a ~2GB file with holes in it. They return almost instantly on OSX.

EDIT: oh wait, this isn't what you want. you want a file _without_ holes, my mistake.