My Cloud Home and My Cloud Home Duo are 2 different devices.
Since you are able to swap HDD on the Duo device, this may change the whole process of Installing Debian.
There’s this thread: Intalling Open Media Vault (alpha state) (finished, there is an easier method now, i keep the post for research data))
I have never tried the method, but It’s more likely to find the answers you are looking for there.
In case you want to Install Debian on a My Cloud Home device,
I’ll be more than happy in guiding you through the process and answering your questions.
