Change matfile on harddrive

Hi.

I am working a project with very much data. At one point I have to allocate space to a matrix that exceeds the capability of my computers RAM. As an example:

    test = zeros(1000000, 14000);

This request cannot be made because "test" takes up 112 GB.

I then thought I could store the matrix on my harddrive and alter it without loading it entirely. Using:

    save('test.mat', 'test', '-v7.3');
    x = matfile('test.mat', 'Writable', true);
    x.test(:, 123) = ones(1000000, 1);

This should change column 123 in the matfile on my harddrive to ones. My problem is that I cannot create the matrix to begin with, because it is too big.

I have then tried using:

    test = sparse(1000000, 14000);
    save('test.mat', 'test', '-v7.3');
    x = matfile('test.mat', 'Writable', true);

This is all good, but when I try to rewrite the file,

    x.test(:, 123) = ones(1000000, 1);

I get the error "Can't write file >PATH<".

Is there any way to save a matrix like the first example without first making it in your workspace? Or is there a way to rewrite the sparse matrix saved on the harddrive?

THANKS :D