  We present a high-density relativistic reflection analysis of 21 spectra of
six black hole X-ray binaries in the hard state with data from \textit{NuSTAR}
and \textit{Swift}. We find that 76\% of the observations in our sample require
a disk density higher than the 10$^{15}$~cm$^{-3}$ assumed in the previous
reflection analysis. Compared with the measurements from active galactic
nuclei, stellar mass black holes have higher disk densities. Our fits indicate
that the inner disk radius is close to the innermost stable circular orbit in
the hard state. The coronal temperatures are significantly lower than the
prediction of a purely thermal plasma, which can be explained with a hybrid
plasma model. If the disk density is fixed at 10$^{15}$~cm$^{-3}$, the disk
ionization parameter would be overestimated while the inner disk radius is
unaffected.
