Did you connect the U3s directly with your computer or with the help of an USB(2)-Hub?

In case you don't already have an external powered USB2-Hub, you will need one (but not sure if one is sufficient). I only have 2 U3s and I run them also with a HUB but I'm not sure if I could connect 7 U3s over this HUB even though it offers exactly 7 USB2-Slots.