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Yes, you need 3 drives for RAID5. You can set up RAID1 and then convert to RAID5 when you have more than two drives, if needed. Your other statements are accurate for RAID5. SHR will allow you to mix drive sizes, but one of the largest drives will be totally reserved for parity. As mentioned, DSM is normally installed to all drives. You can isolate Plex to the SSD if you install it at a separate, Basic volume (obviously no redundancy). I do something like this with my system - 2x SSD's in RAID1 for Docker/Plex, 8xHDD in RAID10 for media files and other storage. You actually can remove the DSM replicas and from specific devices - i.e. HDD's leaving DSM on SSD's, but it's a little obscure (and unsupported) how to do that, and there is no way to reclaim the space that DSM would have used. It's a bit of an extreme tuning strategy and I would not consider it without having at least 2 devices for DSM. If you want to learn more read this: How to manage DSM to specific drives