Hi. I have similiar issues all the time using NSH..since the binaries NSH has including "cp" are usually really old and buggy.
Have you looked at the dsync command..it may work better for you or allow you a workaround.
Yes I have tried dsync, but that just appears to be a wrapper for cp and in any case the sync options dont do what I want. They appear to do a logical OR on a couple of options (file modifciation OR size) where I just want modification really.
I am going to log a bug with BL for this. If indeed cp is broken it should be fixed.