There is also the mount settings. Mostly, USB/SD devices are mounted with the flush option. As a result, the buffered data is flushed to device more frequently, resulting in the "stuttering" behavior you are seeing. I experience this myself when copying big data sets to flash devices. Personally, I don't think there is much you can do about it. Flash discs on the other hand, have controllers that help level the I/O, especially when writing to disc/device.