I cannot answer your question about how to identify the offending device.
I do have some questions of my own though:
What machine are you using?
If it is a laptop, is the laptop running on battery? And if so, is the battery old?
Are you using a USB hub? Is the hub powered?
Are any of the USB devices powered directly? Has the power cord been disconnected? (We have cats …) Perhaps a power supply has died? (Happened, but not because of said cats.)