The NSLU2 out of the box will only let you connect bus-powered USB devices (flash drives, etc) to the Disk 2 port. Its not certain why Linksys limited it in this way, but it can provide power to two devices without obviously breaking any rules. Bus-powered devices are limited to 500mA each, the NSLU2 itself draws about 500mA, and the power supply is rated for 2A, so it should not be a problem.
Linksys actually designed the PCB to power both ports, but disabled Disk 1 by omitting a ferrite bead component in the 5V line. The ferrite bead part is a good thing to have, but not actually essential, so it can be replaced by a simple jumper. The missing part, FB12, is located on the back of the board, under the Disk 2 connector. See the photograph below for the location.
Solder a very small piece of wire between the two pads and you are done! I suggest soldering a longer piece of wire to one pad, then cutting the wire very short and soldering to the other pad.
:I note comments elsewhere that some PSUs? failed when driving NSLU2 + 2x discs. I'm but a rank beginner on this architecture but I wonder if somebody could clarify this- the "Disk 1" interface has a ferrite and will power an external hard drive, the "Disk 2" interface doesn't have a ferrite or a link between the pads but despite this will still power a Flash drive which obviously doesn't use an external PSU. As an experiment I've currently got an RS232? interface plugged into the "Disk 2" interface which shows up on lsusb with "MaxPower? 50mA". -- MarkMLl?
To clarify, the presence or absence of the ferrite really shouldn't make much difference; I would be interested to hear of any device that will function in one port but not the other under this arrangement. The problem with bus-powered 2.5" hard drives is that most of them have to break the rules on USB power consumption and draw more than 500mA, usually when they turn on. That is why many come with those weird dual-port USB cables, so they can draw power from two ports and stay legal. Two bus-powered hard drives plugged into the NSLU2 cannot be expected to work reliably, and will probably fail at power-on.
: When I plugin two Bus powered USB Devices, one of the starts to start, but stops, then starts, this indicates to me it's not getting enough power... But when I switch ports the same drive does it (one would think switching ports would make the other one do it). It works fine if it's the only one or if I plug in an AC Adaptor to it... So as far as I know I can't use two bus powered hard drives? (Maybe if I use a powered hub... [not all my USB Hard drives have AC input).