YouTube video (demo): http://youtu.be/axQvgqvlhcU
The following howto works if you already have USB driver (pre) installed on the device. This can be manually or within a custom image. The HowTo applyies to the most of devices, it uses the TP-Link MR3020 as an example.
Download the image file to your computer, refer to
our hardware list choosing the correct file.
Power off the MR3020.
Take your USB device and copy the downloaded file onto.
Connect the MR3020 to your computer via a Ethernet cable.
Set your computer's IP address to '192.168.1.2', subnet mask '255.255.255.0'.
Start to ping 192.168.1.1 (this is the failsave IP address) infinitely.
Power the MR3020 on and when the WPS Button starts to blink, push it (then it will blink faster). Note: The button name for activating the failsafe mode may vary per device (for example on the 'TP-Link WR1043ND v1': the 'WPS' button is named '
OSS').
You should see the ping getting a response.
Type:
telnet 192.168.1.1
/etc/init.d/boot start (This will return some error messages - ignore these)
mount /dev/sda1 /mnt
cd /mnt/
ls
mtd -r write <your filename> firmware
If the mount does not work, you can do:
mount_root
Which mounts your root partition in order you can edit files. After you have finish, type:
reboot -f
Source (http://wiki.openwrt.org/doc/howto/generic.failsafe)