By exceeding the USB bus bandwidth, do you mean at any given time pushing more data through the drive/cable to the iMac than it can support?
If so, what is that amount of the bus bandwidth limit?
I typically might copy lots of files from a small external USB 3 7200rpm drive to more than one drive inside of the enclosure, folders containing lots of images, files to another drive, and videos to another drive, simultaneously...this doesn't happen all the time, and I wouldn't have a clue as to how much data is being pushed at any given time.
Thank you.