  The key aspect of the very successful truncated disc model for the low/hard
X-ray spectral state in black hole binaries is that the geometrically thin disc
recedes back from the last stable orbit at the transition to this state. This
has recently been challenged by direct observations of the low/hard state disc
from CCD data. We reanalyze the Swift and RXTE campaign covering the 2006
outburst of XTE J1817-330 and show that these data actually strongly support
the truncated disc model as the transition spectra unambiguously show that the
disc begins to recede as the source leaves the disc dominated soft state. The
disc radius inferred for the proper low/hard state is less clear-cut, but we
show that the effect of irradiation from the energetically dominant hot plasma
leads to an underestimate of the disc radius by a factor of 2-3 in this state.
This may also produce the soft excess reported in some hard-state spectra. The
inferred radius becomes still larger when the potential difference in stress at
the inner boundary, increased colour temperature correction from incomplete
thermalization of the irradiation, and loss of observable disc photons from
Comptonization in the hot plasma are taken into account. We conclude that the
inner disc radius in XTE J1817-330 in the low/hard spectral state is at least
6-8 times that seen in the disc dominated high/soft state, and that recession
of the inner disc is the trigger for the soft--hard state transition, as
predicted by the truncated disc models.
