Electronic – Arduino ESP8266: Connecting external power supply and USB from PC at the same time

It sounds like your device needs more than the 100 mA it is guaranteed, but does not enumerated properly to request the extra current. You can get up to 500 mA from a USB port if you ask for it and the host grants it.

There is no guarantee that the host will grant the request, but desktop machines pretty much always do since they have plenty of power available. In fact, the desktop motherboard schematics I have seen show only a polyfuse between the 5V supply and each USB port. There is no active electronics limiting current. On those machines, you get the full 500 mA even if you don't ask for it.

Laptops however have limited power and therefore actively monitor and control USB power in most cases. Some machines may limit the sum of all USB devices to a certain current, or limit it more when running on battery only, or when the battery is low, etc. Note that USB powered hubs can't ever grant the full 500 mA since that is all they are getting themselves.

The reason your device won't enumerate anymore is because you broke the ground connection. All three lines D+, D-, and ground are required for communication. You can cut the power line if the device is self-powered. Self powered devices generally have no connection to the USB power line. That's perfectly OK, but they still have to connect via ground.

That is super terrible design... but will it break anything? Good question..

If you try to backfeed a voltage into a supplied voltage, the difference in the two will determine the amount of current that flows. So if the supplied current is 5.1V and the applied voltage is 5.2V, 0.1V will cause current to flow into the 5.1V supply, unless it's specifically designed to prevent current from flowing backward. (this would depend on how the switching converter is designed)

So say you have a long cable, and 0.1V of mismatch... that might be around 5 ohms... That is 0.020A, or 20mA. That is enough current to run an LED, but usually not enough to do damage to a power circuit. A much worse condition (as Russell points out) is when the computer is OFF and there is power being backfed.

Most USB power systems are going to have a resettable fuse somewhere around 500mA to 1.5A, so that would protect things in an extreme case, but the real answer is that it depends on the computer, and it depends on the design.

My best advice is to stay away from USB hubs that are of mindblowingly poor design.