thoughts after install - usb cut out

[namtih]

It's no issue of xbian. It's more an limitation of the Raspberry hardware and I made the same experience some weeks ago as I got my unit.
The problem is not the power supply, the problem is more the interface connection between the power supply and the USB ports. I think the USB ports only delievers around 300mA which could be too low for some devices.
So when you hot plug the device there will be an peak which cause the Pi to restart.

You could try to power off the Pi, connect the USB device and then power on the Pi again. See if it delievers enough power to the USB port to get the drive working. For me it didn't worked as the drive didn't spin up.

I ended up with using an external usb hub with external power supply.

If you need more information, you can use Google with "raspberry usb device restart" or for example this thread:
http://www.raspberrypi.org/phpBB3/viewtopic.php?f=28&t=17611