I thought I'd share my own benchmarking since you had asked about Seagate Momentus 5400.6
This is a pic of HDTach with the HDD connected to SATA port on the laptop.
I've build my own portable drive with separately purchased 2.5" external drive enclosure.
Here is the HDTach reading of the same drive connected to the USB 2.0 port
[pretty sad ]
IMO the bottleneck is the SATA-to-USB controller on the external enclosure.
If the pre-build ones listed have better performance, then I'd suggest to buy that.
On the other hand if you find any review of those listed as the same performance, then I'd suggest you to build your own.
It's fun to build your own and cheaper too.