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OSX:
http://osxdaily.com/2013/05/31/create-large-file-mac-os-x/
mkfile -n size[b|k|m|g] filename


Others:
http://www.skorks.com/2010/03/how-to-quickly-generate-a-large-file-on-the-command-line-with-linux/
dd if=/dev/zero of=somefile bs=1 seek=1G count=0