USB 2.0 theoretical speed is 480 Mbps Mega bits per second, not 480
MBps Mega Bytes per second.
Real speed never gets close to the theoretical speed, for one thing
there is way too much overhead of the communication hand-shaking.
480 Mbps you might think that divided by 8 (for 8 bits per Byte) you
should then think the theoretical speed would then be 60 MBps, but
with the over-head it is more like 10 bits per Byte, so that gets to
be about 48 MBps ... THEORETICAL.
Getting a fourth to a third of the theoretical speed (12 to 18 MBps)
is much more realistic, and you are getting 15 MBps, so you are right
in the ball park.
I have a high speed flashdrive (not all are high speed, even though
they are rated for USB 2.0) and I get just about 1 GBpm (Giga Byte per
minute) transfer speed, which is just about 17 MBps. I am pretty happy
with that.
