error: file 'data.nii' smaller than expected


I’m trying to follow this (ISMRM tutorial - Structural connectome for Human Connectome Project (HCP) — MRtrix 3.0 documentation) tutorial and under ‘diffusion image processing’ step 1 I’m having some trouble running the command and the error message I get is ‘file ‘data.nii’ is smaller than expected’. How do I overcome this problem? What could be causing it?

The command seems to work fine for other files of the exact same size but for some files I get the same ‘file ‘data.nii’ is smaller than expected’ error.