Very interesting - did not know that. 
james
The high end disk enclosures used in enterprise level systems (and we are talking big bucks here) are all fronted by lots of cache memory, which means (in most situations) that when data is sent to the harddrives, it only needs to hit the cache (ie the read or write is satisfied immediately) and then is "destaged" as needed later on.
Also they contain many harddrives which use special techniques (such as striping, hotspot migration etc) so that a single drive doesnt get saturated.
But your consumer based enclosures dont have this, so your speed is determined by the small amount of onboard memory the drives has. The saturation point of the drives in a consumer based enclosure is very small.
As with all things... speed costs money.
Peter
PS... and aside from onboard cache, you have rotational speed diffence between different disk drives (5krpm, 7.5krpm, 10krpm, 15krpm) which alter throughput characteristics
PSS...and you have all the handoffs occuring...if I understand the situation... PC reads data from its disk (bottleneck 1)... stages this into the network subsystem on PC (bottleneck 2)... transfer to the BDP across the network (bottleneck 3)... BDP stages this from its network subsystem to the USB subsystem(bottleneck 4)... then writes to the BDP attached drive (bottleneck 5)