iMac USB problems.

I'm having some USB power problems with my iMac 20" (late 2006)

So to start, my iMac has 3x USB 2.0 ports of which:

• 1 has my 3.5" eternal HDD plugged in (it has its own power)
• 1 has an extension plugged in so that I can plug in flash drives
• 1 has a 4 port USB Hub plugged in. It doesn't have its own power source. plugged into this is:
  
• a printer (off most of the time)
• iPod shuffle dock
• iPod dock
• Keyboard (mouse plugged into keyboard)

I havent experienced problems in the past. I can plug in two iPods into both the docks and not experience problems but the problem comes in when I plugged a 2.5" external HDD into the USB extension. The problem was that I couldn't use the mouse. I had full keyboard control and the mouses' optical light was still on. It was just unresponsive on screen.

Is my USB setup underpowered? or is there some other problem?

Also; is USB power given per port or does the entire system have a limit?

Ok, since no one knows about my problem how about a simpler question:
Does anyone know anything about the USB spec of the late 2006 iMac's?