Question: I have a new (2013) 15" Macbook Pro.

The USB port provides 500mA which is not good enough for many devices I use (Hard Drive, 3G Dongle...)

Reading this article: <http://support.apple.com/kb/HT4049> it seems that these ports are capable of delivering more power, but it's limited to Apple products.

Is it possible to hack the mac, and change this default value (to something like 900mA?)

Answer: 
Apple USB 3.0 ports will output up to 1100mA if requested, USB 2.0 is limited to 500mA

You can check the current requirements for any attached device in Apple Menu > About this Mac > More Info (later macOS versions now labelled 'System Report…')... USB

I only have USB 2.0 ports on this machine, but see pic...

![enter image description here](https://i.stack.imgur.com/cOovo.png)
Another possible solution, that avoids any sort of hacking, would be to use a USB-Y cable. These cables provide two usb connectors that plug into your laptop and merge to a single cable that's plugged into your external device, therefore pulling current from two usb ports on your laptop. Many external HD's come with these, they're inexpensive, and do the job. See <http://www.toshiba.com/us/accessories/Cables-Adapters/Cables/USB/BA-82010> for an example.