I'm pretty sure you need a 1MB file of a's, perhaps a Gigabyte, why not a Terrabyte.
I have no idea how long it would take to allocate a Terrabyte or if your computer can actually handle it.
pop is a ruby script, for creating files of a specific size. Add it to your dotfiles or check out mine.
Just run pop and pass in a size and file
$ pop 1MB file.txt
> Populated file.txt with 1048576 bytes
$ pop 1.15GB file.txt
> Populated file.txt with 1342177280 bytes
Invoke help with pop -h.
$ pop -h
> USAGE: pop <size>[,B,KB,MB,GB] <filename>