The HyperOS HyperDrive5 (64GB max: $450) and ACARD ANS-9010 (64GB max: $400) are two examples that come to mind. I expect you could use eSATA or put them in a USB-SATA drive case.
I guess those are the closest implementations, gosh they're big!
Maybe serializing the interface they could get smaller: those units just pack some standard ddr (or the like) modules into some adaptor, if I get them right.
@Curaga: interesting, I had no competence to evaluate that point.
I understand that usb is one of the communication protocols relying the less on hardware and the most on software, is that arguable?
I know of a bunch of overlapping standards for data over a net: AoE, iSCSI, FCoE.
Sadly they all pass through an interface other than usb.
The wonderful page (in which I feel close in spirit with TC principles in many ways):http://harmful.cat-v.org/software/
promotes the first as the least harmful option =)