Citação do artigo sobre USB da Wikipedia:
According to routine testing performed by CNet, write operations to typical Hi-Speed hard drives can sustain rates of 25–30 MB/s, while read operations are at 30–42 MB/s; this is 70% of the total available bus bandwidth. For USB 3.0, typical write speed is 70–90 MB/s, while read speed is 90–110 MB/s.
Por isso, parece que a conexão USB 2 é o gargalo.