By dynamic light scattering we have studied suspensions of ferrimagnetic maghemite (gamma-Fe2O3) nanoparticles in n -decane with attractive interparticle interaction. The measurements in the suspensions of different concentrations ranging from 0.21 to 25.8 wt % have been compared in zero external field and in the magnetic field of 270 mT. In all samples well-defined relaxation process was observed. In the absence of an external field the suspensions were homogeneous, while in the magnetic field most of the suspensions undergo phase separation in a needlelike islands of a very dense suspension surrounded by a dilute suspension. The underfield dynamical behavior is found to be anisotropic and diffusive in both directions at low concentration. For the more concentrated sample, in the direction parallel to the external field, it remains diffusive with a larger diffusion coefficient while it is not so in the perpendicular direction, in which the mean-squared displacement grows faster than linearly with time and the dependence of the relaxation rate on the scattering vector q is not quadratic. In this direction the dynamics of the system present similar features as glasses or gels close to the dynamical arrest.