Most run of the mill laptop hard drives are only 4200RPM with transfer rates of only 15-20MB/s. 5400 RPM hd aprox 25MB/s & 7200RPM upto 35MB/s.
Please ensure that UDMA is on for your HD. Otherwise your HD will be painfully slow.
Also ensure that you have installed the latest USB updates from microsoft (service pack 1 & above).
USB is very cpu intensive, it was never really designed for high data throughput, the interface was designed for small bursts of traffic. USB2 was a speeded up version of an unsuitable interface.
Given that 4x is approximately 5.5MB/s, 8x is 11MB/s, perhaps the HD in the laptop simply cannot keep up with the writer, especially over a USB interface & if UDMA is not on for the HD.
Also, what is the CPU speed of the laptop?