  We examine the HI-based star formation efficiency (SFE_HI), the ratio of star
formation rate to the atomic Hydrogen (HI) mass, in the context of a constant
stability star-forming disk model. Our observations of HI-selected galaxies
show SFE to be fairly constant (log SFE_HI = -9.65 yr-1 with a dispersion of
0.3 dex) across ~5 orders of magnitude in stellar masses. We present a model to
account for this result, whose main principle is that the gas within galaxies
forms a uniform stability disk and that stars form within the molecular gas in
this disk. We test two versions of the model differing in the prescription that
determines the molecular gas fraction, based on either the hydrostatic
pressure, or the stellar surface density of the disk. For high-mass galaxies
such as the Milky Way, we find that either prescription predicts SFE_HI similar
to the observations. However, the hydrostatic pressure prescription is a more
accurate SFE_HI predictor for low-mass galaxies. Our model is the first model
that links the uniform SFE_HI observed in galaxies at low redshifts to
star-forming disks with constant marginal stability. While the rotational
amplitude Vmax is the primary driver of disk structure in our model, we find
the specific angular momentum of the galaxy may play a role in explaining a
weak correlation between SFE_HI and effective surface brightness of the disk.
