I don't think you have a problem. Copying from one partition on disk A to another partition on disk A will be slow. The arm has to move all the time. It would be faster if you moved the file.

And if you have a second internal disk, it is probably faster to copy from C to the other disk and then from the other disk to D.

Regarding your flash drive you cannot go by the maximum capacity of the USB2 port. You also have to know the speed of the USB stick - and some are extremely slow. If you want to know exactly, run this program. Let it run to the end and you will have the data.

http://www.hdtune.com/