The marketing materials describing transfer rates refer to the speed of the bus, not the disk itself. The following are the nominal transfer rates of each bus:
- eSATA (first generation): 150 MB/S
- eSATA 2: 300 MB/S
- eSATA 3: 600 MB/S
- USB 2.0: 480 mbit/S
- USB 3.0: 5 gbit/S
The bus transfer rates determine the limitation of the bus speed, not how fast your computer, memory, other disk, etc. A single disk mechanism can only reach about ~90MB/S sustained throughput regardless of bus- the nominal bus speed only determines burst rate, which is the initial momentary transfer. All other factors inbetween can only serve to slow down the transfers further.