1st. Win7 has a silly way to calculate file transferring speed. It shows the progress of data migration to memory buffer instead of the final destination.
2nd. This bug is very obvious with larger files when the source drive has faster copying speed than writing speed of the destination drive, it hangs at 99% (or before finishing each file in a sequence) while waiting for data to finish writing from memory buffer.
Have a look at the other USB devices you have connected; My cellphone for instance causes some screwiness when I have it plugged in.
Some USB devices do not "play nice" with others, you better not use them simultaneously, specially on the same USB hub.
4th. Try copying to a different hard drive (not sure about 11-12MB/s, but if the one you are using is near full it's writing speed may drop below 30MB/s) or using a different USB 3.0 reader.