Category C (Class 2) LGV / HGV training runs around Ipswich, Suffolk and uses the DVSA Test Centre in Ipswich. C is the category that allows you to drive a rigid vehicle over 3,500kg.
USB 2.0 is much faster than the DVD data rate. DVD 1x (slowest) write speed is 1.39MB/s. DVD 24x (fastest) write speed is 33.24MB/s. USB 2.0 write speed is up to 35MB/s. A 24x DVD max is about 32MB/s while USB2 max is about 60MB/s.
The Serial ATA drive and interface are faster than USB 2.0, so the USB 2.0 drive and interface are the bottleneck. It may be that the destination disk and the interface can move and write data faster than the source disk can send it.