  We report high-field magnetotransport measurements on
\beta''-(BEDT-TTF)_4[(H_3O)M(C_2O_4)_3]Y, where M=Ga, Cr and Fe and Y=C_5H_5N.
We observe similar Shubnikov-de Haas oscillations in all compounds,
attributable to four quasi-two-dimensional Fermi-surface pockets, the largest
of which corresponds to a cross-sectional area ~8.5\% of the Brillouin zone.
The cross-sectional areas of the pockets are in agreement with the expectations
for a compensated semimetal, and the corresponding effective masses are ~m_e,
rather small compared to those of other BEDT-TTF salts. Apart from the case of
the smallest Fermi-surface pocket, varying the $M$ ion seems to have little
effect on the overall Fermi-surface topology or on the effective masses.
Despite the fact that all samples show quantum oscillations at low
temperatures, indicative of Fermi liquid behavior, the sample- and
temperature-dependence of the interlayer resistivity suggest that these systems
are intrinsically inhomogeneous. It is thought that intrinsic tendency to
disorder in the anions and/or the ethylene groups of the BEDT-TTF molecules
leads to the coexistence of insulating and metallic states at low temperatures;
comparison with other charge-transfer salts suggests that this might be a
rather general phenomenon. A notional phase diagram is given for the general
family of \beta''-(BEDT-TTF)_4[(H_3O)M(C_2O_4)_3]Y salts, which exhibits
several marked similarities with that of the \kappa-(BEDT-TTF)_2X
superconductors.
