I was searching for the same issue as you have.
I just bought a External USB SATA casing and put a 500Gb drive into it.
My plan was to use it as a combined portable storage, portable Ubuntu.
The problem I have is that in windows the drive functions perfectly, I can use it I had it connected for hours with no issue.
When I try to Install Ubuntu, it goes through the entire installation fine.
I then reboot and start booting up the drive, and it will freeze up on random, but the same thing happends everytime, the Harddrive indicator Light on the External Casing goes off, Well it is still spinning, but the Light on it indicates it has been shut down, then Ubuntu will hang. there is no way of getting it up and running again, If I power cycle the drive with the switch on the drive, the light will come back on, but Ubuntu is still frozen, unplug, replug does no difference, light stays off.
Only thing to do is to Power cycle Harddrive and Reset PC.
My drive is a combined USB2/Firewire800 drive.
And when I used it on My laptop with Kubuntu, it functioned fine for hours running on Firewire, I have no USB2 on that one.
Unfortunatly It can not boot from Firewire.
On my Desktop I have USB2, but the firewire is on soundcard and is therefore not available during boot.
I will try to install Ubuntu on the desktop PC and have it connected on USB2 while having Ubuntu running on another drive to see if I can reproduce your issue.
If it does the same it might be that there is a bug with External SATA drives on USB.
Will come back with my experience.