This is how i started to test alternative firmwares on the WD My Cloud.
You need a modified uRamdisk. (https://github.com/Johns-Q/wdmc-gen2/raw/master/uRamdisk) is a already a modified uRamdisk from FoxExe.
(Many thanks for his work.)
It looks for a 2nd partition on the usb stick and boots it if possible.
When nothing is found on the 2nd partition, it boots from the harddrive
when there is already an other firmware installed and when this is not working it starts the ssh daemon.
See Clean OS (Debian), OpenMediaVault and other "firmwares", but extract the jessie-rootfs.tar.gz to /dev/sdb2.
I can make a step by step howto, if you need it.
Johns
