We have a lot of cloudy today.
I tested the frame rate of my newly arrived USB 2.0 on my slow Sony Vaio U3, I could get around 12 fps when the built-in hard drive is used to store the AVI.
Then, I tried to use something else which should be faster than my hard drive, what I used was a 2G SD card inside a USB 2.0 card reader. I guess a fast USB thumbdrive should perform better, but at least it should be faster than a mechanical hard drive.
The result is 7.4fps... surprised? Not really.
With the DMK 21AU04.AS, the USB 2.0 bus was nearly saturated, especially given a slow CPU. The USB 2.0 specification is far more CPU demanding than that of the Firewire interface, and therefore, the DMK already sucked up all the juice.
To add one more USB 2.0 device to the same bus will only make the situation worse.
Therefore again, if you have a slow computer like I do, go for Firewire version, it will offers better performance as my former experiments indicated.