Currently, I have a USB cable that connects from my Macbook Pro to my Dell Monitor which has a built-in USB Hub. On my monitor, I have a Apple Universal Dock connected and a WD My Book external harddrive. Whenever I connect something light as my 3rd usb device on the monitor such as a thumb drive, or connecting my camera to import photos, it runs perfectly fine. Strangely, I connected a newly bought WD Passport drive as a 3rd USB device and I get an error that says something like "USB hub is drawing too much power...will disable now". I was wondering how much power can these USB handle? How many external harddrives max can I connect via USB hub? Importantly, does it make any difference if I connect 2 WD My Book drive compared to 2 My Passport drive? (the difference between the two is that the My Book drive is powered by a cable that connects to the power outlet.) What I want to eventually do is have 3 external harddrives connected via a USB Hub.