I have the same issue. Since the 5660 doesn't have any way to have 2 HD's installed at the same time, I've been looking for an enclosure to hold an external laptop (2.5") drive.
I don't like the idea of having to plug the enclosure in to A/C power just to use it with my laptop, which would be a requirement with any Firewire enclosure. So, though it won't give quite as good overall throughput, I'm planning to go with a USB 2.0 enclosure for an external drive.
I found one by Utobia for $44 that claims to power the drive off the current supplied through the USB connection. Of course, this would no doubt eat battery, but if I'm using my laptop on battery power, then plugging in an external HD enclosure is probably not an option. At least having a 2nd battery installed in the 5660 *is*.
And, of course, there's the added benefit that this enclosure would also hook up to, and work with, a USB 1.1 connection, too, so if you ever wanted to use it to share some files with somebody else who has a PC, you're almost certain to be able to, whereas most people don't have Firewire ports on their PCs.
Sorry, I don't remember right now which website I found the Utobia enclosure for sale, but I just used Google to do the search that eventually led me to it, so I'm sure any of you can do the same. FWIW, I did find one or two enclosures that had both FW and USB2.0. I think the cheapest was around $85, but I'm not sure that any of those had the option to run off bus-supplied USB power.