It still isn’t clear what “2 pcs” is referring to. Do you have two My Cloud units? Or are you asking about trying to set up disk swapping on one or more external USB hard drives attached to the My Cloud’s USB port?
If your changes are being overwritten upon each restart or power on then you need to look into using CRON to modify the firmware post boot up if using 2nd gen version 2.x firmware. For older first gen version 4.x firmware one can try and use the /etc/rc2.d/S98user-start file.
Here is one example of running a boot script on the 2nd gen version 2.x firmware units:
https://community.wd.com/t/how-to-run-a-user-boot-script-at-mycloud-gen2-2-11-xx-devices/169822
