I can get full 16x on my laptop with a Seagate FreeAgent Desktop 250 GB USB hard drive or my laptop’s internal hard drive & either of my USB burners.
If I switch to my desktop, however, the USB performance is cut in more than half to 6x, maybe 8x on occasion. I don’t use that setup often.
Are you using a laptop or desktop, how old is it, and where did you read it was better doing external-to-external? That’s not always the case.