I've had issues with USB devices drawing more current then the port could put out. Without a power meter, it looks like the device just doesn't work (although t

I've had issues with USB devices drawing more current then the port could put out. Without a power meter, it looks like the device just doesn't work (although the Linux kernel logs typically show a device disconnect event). With the meter, you can see a current peak up to a nice round value, followed by a voltage drop. The low refresh rate makes it not nearly as clear as with an oscilloscope, but once you know what to look for, it is easy enough to pick up. (And if you really want to, you can get one with a breakout to connect scope probes). If this is an issue you run into often, I'd also suggest a usb variable load tester to check exactly how much current a port will output.