How much current can be drawn from the three USB sockets on the rear of the RipNAS? And are there any limits to which ways the current can be drawn eg. if it's (say) 200mA from each port, can I draw 600mA from just one port if I don't draw any current from the other two? Are there any problems with peaks for initial spin up currents? I'm using the external 12V PSU that came with the RipNAS.

I may be wanting to attach USB powered 2.5" external hard discs to the RipNAS for backup purposes. Hence my question.