Hi Dan
I don't have any experience with the Garmin Edge, but it sounds like the iMac's USB bus was or is overloading and tripping the port overload protection.
A few question if you don't mind.....
1. Is that Belkin hub self-powered (having it's own power supply) or bus powered?
2. What happens when you connect the edge directly to a USB port on the back of the iMac?
3. How many other USB devices are connected to the iMac and briefly in what configuration?
Also note: that instead PMI or SMU the correct SMC reset process for an Intel iMac is as follows:
http://support.apple.com/kb/HT1543
Dennis