OS 5 downgrade to OS 3

My Cloud OS 5 Personal & Network Attached Storage Product Ideas
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At this point, call OS 3 an upgrade, I’m still actively looking for a way to safely upgrade my PR4100 to OS 3 before this OS 5 ■■■■ chews through my hard disks

Edit: Saw this post in another thread, please note that I’ve not tried this yet.