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?)
You can fix this by using a powered USB hub. This way you do not have to modify your mac.
This Belking 4-Port USB hub for example.
The only negative, you need a power socket.
Another fix is to use a Thunderbolt hub, like the Matrox DS1. This is a hub with Thunderbolt input, needs no additional power, and outputs all sorts of IO, including USB 3.
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.
The USB port provides 500mA which is not good enough for many devices I use (Hard Drive, 3G Dongle...)
The USB 3.x ports on Apple computers are able to supply more than 500 mA. This can be demonstrated by plugging in an iPhone and see the computer report in System Information that it is supplying 12 watts. The ability of the port to supply power doesn't change with what is plugged into it.
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.
The power the port can supply is not limited by what is plugged in. This document is not intended for a highly technical audience so it's in a way lying by omission.
Is it possible to hack the mac, and change this default value (to something like 900mA?)
Much of this default behavior is written in the device, not the host. And Apple computers built after iPods started using USB for charging (2005 or there about) will provide at least 1500 mA from their USB ports.
You don't have to "hack" anything for it to provide 900 mA to a USB device. The USB 2.0 and USB 3.x spec allows for up to 1500 mA to devices. Apple computers since 2012 or so were built to provide 2400 mA from USB. Apple isn't doing anything "sneaky" or out of spec in providing this extra current from USB ports to Apple iDevices. They use the USB-PD and USB-BC protocols for this, and other USB devices can safely use this power too if they use the same protocol.
Few USB devices will require more than 900 mA from a USB host because for a number of reasons few USB hosts provide more than 900 mA. Apple computers will happily provide this much power without any "hack". Because this budgeting of power relies as much on the device as on the host there's ways to get more power by "hacks" to the device. That's assuming one desires well behaved USB devices. It's possible, and trivial, to create a device that will take 12 watts from a USB port like an iPhone would but without asking nicely first like an iPhone would.
