Powering USB device from arduino uno? I'm wanting to power a USB device from an Arduino Uno. Forgetting about the two data pins, it just requires power. Using a power meter, I can detect the 5v pin at about 4.95 volts, however I don't have access to any resistors at the moment so cannot test the amperage because it just shorts.

The USB device I want to power requires ~300mA (can't remember specifically). First of all, does the Arduino get 100mA or 500mA when getting power from the USB? Is this changeable depending on the setting on the USB Host (High 500mA, or Low 100mA)? 

Secondly, how much would actually be usable? I assume the Arduino itself would require some of that current? Even more if a program where running on it at the same time? (I have no idea how computers utilize power) I would like to just hook up my usb device to ground and 5V with whatever resistors (and any other electrical components) in between to bring it down from 500mA to the required ~300mA.
You'd have to test this but there's a chance you could supply the Arduino using a DC power supply on it's 5V pin. In this case DO NOT TURN YPUR SUPPLY HIGHER THAN 5V, as I don't believe there is any protection and will send the full voltage to the USB.  

A safer option would be the Vin pin. Normally you'd want to put 7-12 Volts to this pin but you may want to start at 5V and work your way up just to be safe.

Someone please corrects if I am wrong, but if you never apply more than an external 5V your worst case scenario would be burning out the Arduino but not the USB device.