The NEC USB2 chip used in the slug supports five USB2 ports in total. Two of these are the regular USB-ports at the back of the slug, but the three additional ports are also enabled. The traces and pull up resistors to these ports have been traced by Attila Csipa, and all of the additional ports have been verified to be working.
You can use these ports to connect additional USB devices without using a hub, or to replace a fried port on your slug.
To connect an USB device to one of these ports, you will need to solder four wires:
Ground is easy enough. You can take that from the closest USB-port. Power is more difficult. I pulled this from a broken USB-port on my slug, but you can't do this with a working port as you are limited on the amount of current you can draw per port (500 mA), so unless you are replacing a broken port it is highly recommended that you do not connect bus powered devices to the extra ports, only devices that have an external supply (printers, some external 3.5" HDD racks, etc). Please take into account that each active high-speed port also consumes an additional 50 mA. The three ports end up in the resistors R160 to R166 (R162 is missing).
R160/R161 is port 3 (D- and D+ respectively)
R163/R164 is port 4 (D- and D+ respectively)
R165/R166 is port 5 (D- and D+ respectively)
Note that the NEC chip has two OHCI controllers, which means it groups the USB ports into two groups for low and full speed USB devices (this might be important if you managed to fry one of the controllers with all its ports)
Group 1: ports 1, 3 and 5 Group 2: ports 2 and 4
I've used R163/R164 and R165/R166 to add two ports to my slug, one powering a 200mA memory stick, the other powering a 100mA PL2303 USB->Serial converter.
Looking at the picture above, I connected D-/D+ from the memstick to R165 and R166, on the side facing RP1/RP2 (Away from the USB-port).
I've done the above modification and it worked well, pictures of my new 4 USB NSLU2 are on my gallery: http://www.sdwphotography.com/gallery/v/house/nslu2-hdrfc SteveWormley January 02, 2007, at 06:25 PM
It would probably be possible (and easier) to add a female USB cable through a hold drilled in the back.
I used this guide to put a sandisk 2 GB pcb inside the slug as an internal drive to hold my debian OS.
I've placed a 1GB flash pcb inside the casing (removed the shell), connected it to 'port 1' (where formerly USB1-connector was), and I've connected the USB1 connector to port 3. Debian works fine, I've successfully tested a usb-camera on USB1, don't know about higher loads (such as usb-powered hard disk drives).
I've pulled off the two usb ports, and replaced each of them with a stacked pair pulled from some old motherboards. the pinouts for the lower port, and the outer grounding tabs are the same as the slugs single ports. cut off the inner pair of grounding tabs, and cut a window in the back of the shield to fold up the 4 leads for the upper port. you can solder the ground pin right to the shield. make sure the other pins dont touch the shield. i routed the power right to the + side of the power jack. oh- and if you accidentally remove on of the pull-ups, they are only 37 ohms, so just bridge the gap with wire. -- kitno455
I have one slug with internal USB flash stick, and one with embedded SDHC card reader. The SDHC version is very nice. http://panola.kyla.fi/~jap/NSLU2/
I used this information to add a small 4 port hub to my slug. I purchased a cheap 4 port USB 2.0 hub and unsoldered the USB and power connectors. +5v for the hub comes directly from the slug's DC-in jack, and the hub's USB data lines connect to one of the spare USB ports. Hardly elegant; but it avoids the need to tap power from one of the existing USB ports.
If your're still unsure where to wire what, have a look at some very detailed photos from my soldering here: http://www.direcs.de/nslu2/index.html Your can also see, where to put the additional USB ports.
See also more info on USBSlotsAndBuses