I've got a USB 2 drive as a network share on the same pc as the portable one I just bought, it's fast enough, but I'm generally not trying to write crazy amounts of tiny files to it at one go.

Plugged in the new external drive to my home pc this morning and it wouldn't mount. I could hear an ominous clicking... Turns out the new-fangled USB 3 drives need more power than a USB 2 port provides (per mfg's web site faq), so I plugged it into a powered hub I happen to have for travel, and it works fine.

This morning I was unable to mount the portable drive's nfs share on my mac, so I nfs-mounted the mac source hdd on the ubuntu pc (where the portable is plugged in), and am running the copy from there, hitting >40 MB/s!