Weak attractive interactions lead to a substantial broadening of the fluid-crystalline coexistence region of crystalline, colloidal hard-sphere dispersions. Here, we demonstrate that a weak depletion attraction introduced by nonadsorbing polymer added to the continuous phase results in a strong decrease of the low shear viscosity even at particle concentrations up to Phi = 0.65 analogous to the re-entry phenomenon in highly concentrated glassy systems. This promotes flow in macroscopic channels but the scenario changes in confined, microfluidic geometries. Pure crystalline suspensions exhibit flow induced melting in the region of highest shear rates close to the wall, and this promotes flow through microchannels (< 100 mu m). In contrast, weak attractive interactions fortify preferential crystallization at the channel walls finally resulting in channel clogging and cessation of flow. This wall crystallization phenomenon particularly impairs viscosity determination from classical parallel plate rotational rheometry if the gap width is set too low. These findings may find application in the important biotechnological field of protein preparation and crystallization. (C) 2014 The Society of Rheology.