Extroot - tobiaswaldvogel/openwrt-ib42x0 GitHub Wiki
Howto: Install packages to an usb stick or to disc
The space in the flash memory is limited and you might need some extra space for big packages. OpenWRT writes all deltas against the read-only squash filesystem to an overlay, which can be moved to an external USB stick.
Usb stick preparation
Usually if you have already two discs installed then you USB stick is recognized as /dev/sdc. First use fdisk to create a partion. Remove all existing partitions and create a new primary partition. Now create a new filesystem, I recommend ext4:
mkfs.ext4 /dev/sdc1 -m 0
Afterwards mount it somewhere
mkdir /tmp/usb
mount /dev/sdc1 /tmp/usb
Copy the existing overlay
cp -a /overlay/* /tmp/usb/*
Add as extroot
In the web interface add an entry in System -> Mount Points and mark the Option "Extroot"
Important
After flashing a new firmware your USB will be mounted in /tmp/overlay-disabled In order to activate it again remove the md5 sum file. Anyway you should re-install the packages as probably they were updated.