You'll never get 480mbps from a USB-attached disk, even directly connected to the Mac. The disk drive itself simply can't pump out the bits that fast.
Unfortunately, hard drive manufacturers (such as Seagate) don't publish the internal disk transfer rate for consumer ATA and SATA drives. But they do for server SCSI and SAS products. The internal transfer rate is a measure of how fast the bits can be read off the disk platters. Seagate's Cheetah line of 10,000 RPM SCSI disks have a published sustained transfer rate of 39 to 80 Mbytes/sec, which is likely to be much higher than for the consumer-grade SATA and ATA drives.
Compare that to 480mbps, which translates to a raw speed of 60 Mbytes/sec, which seems to fall in the middle of the SCSI transfer rates. But a statistical analysis of the USB protocol shows that actual USB speed is only about 10-20% of the raw speed, due to the overhead and delays inherent in the protocol. This puts USB 2.0's top speed at around 12 Mbytes/sec, more or less.
Analysis of the USB 1.1 protocol:
http://www.edn.com/archives/1996/102496/df_01.htm
Speed tests of USB 2.0 and Firewire:
http://www.digit-life.com/articles/usb20vsfirewire/
USB 2.0's raw speed of 480 mbits/sec was carefully chosen to appear to be faster than Firewire's 400 mbits/sec, but theoretical analysis and practical tests show the actual throughput is much less. That consumers believe USB 2.0 to be faster is a testament to the power of marketing and branding.