Next step, I want to copy the system file into related partition.
Can anybody tell me how to do this step?
You can write the system OS image like JRman wrote is the easiest and fastest method.
If you are a GUI guy (like me) you can write it using a GUI tool like i said in the other thread download the firmware unpack the system OS file rootfs.img and use disk utility from your ubuntu ( i did in lubuntu but it should be the same).
Unzip WDMyCloud-030104-139-20131028.zip to get sq-030104-139-20131028.deb.
Unzip WDMyCloud-030104-139-20131028.zip to get data.tar.
Unzip data.tar to get a folder named CacheVolume.
Inside CacheVolume there is another folder named upgrade.
Inside upgrade there is a bunch of files the needed is rootfs.img
Open disk util in ubuntu (disk, disks, gnome-disks several names…just alt-f2 "gnome-disks) select the first raid partition in MyCloud HDD , click on the gears icon (the one in the middle of the util screen)
click on “Restore Disk Image”, browse using the combobox and select rootfs.img.
Click open and finish the write process, repeat these steps with the second raid partition, close util, shutdown system, reconnect HDD to miniboard, ethernet cable to router and power supply.
If Partition layout is ok and hardware is ok this should recover a soft brick in MyCloud.