Welcome -> Home -> DD-WRT experiences -> Micro & Mega  
Thanks to user lightix from the DD-WRT forums, it is now (jan, 2009) possible to have a Micro version firmware installed on an ASUS router and a Mega version firmware installed on a USB drive.   After boot the router will run the DD-WRT version installed on the USB drive.

Download this file:  micro_plus_usb_asus520gc.bin  - and install as firmware on the router.
Download this file:  mega.tar.gz - and unpack on USB drive. The partition must be formatted as ext2, and be the first partition on the USB drive. Partition must be bigger than 15Mb.

You can also create these files yourself, just follow lightix's guide at the dd-wrt forum thread.

last edit 05-01-2009