That may be your problem. One may have to find a way to roll back to the OS3 (v2.x firmware) for their device before attempting to run the steps to try and install Debian. Because you are both using OS5 and using a different device than the single bay My Cloud (the general subject of this My Cloud subforum/discussion thread) you may (if you haven’t done so already) want to see the dedicated OS5 EX Series subforum where there is discussion on the OS5 EX2 Ultra.
OS5 My Cloud EX Series
From there you can use the forum search feature (magnifying glass icon upper right) to find discussion on attempts to roll back to the OS3 firmware. For example see the following discussion in that subforum.
