Dawn-dusk asymmetries in the ionosphere may be controlled by the interplanetary magnetic field or by internal magnetospheric processes. We investigate this control by inspecting the occurrence of fast azimuthal flows in the nightside ionosphere, using an 18-year database of ionospheric radar data. We find that 80% of the flows display the expected IMF By dependence, that is, eastward for By > 0 and westward for By 0. This rises to 87% when considering only those nightside flows above 62 degrees magnetic latitude, dropping to 44% when considering only the lower-latitude flows. We compare these occurrence distributions to the IMF clock angle, auroral electrojet index (AE), and total ionospheric transpolar voltage (Vpc), and find that the By-dependent flows tend to occur during weak solar wind driving, when the IMF is moderately northward, the AE index is low (AE 100 nT) and the transpolar voltage is modest (Vpc 50 kV). When the convection pattern expands to the mid-latitudes under stronger driving conditions the flows become predominantly westward, irrespective of IMF By. We suggest that this is due to the effects of processes in the inner magnetosphere that are intrinsically asymmetric, such as the Harang discontinuity and sub-auroral polarisation streams (SAPS), overriding any IMF By influence on the convection pattern morphology. We also find some evidence to suggest that intervals of non-IMF By controlled fast flows persist for longer (up to 120 mins) than the By-dependent flows (up to 60 mins).