According to the dx517 specs on the Synology site:
“To have better performance, we recommend creating storage volumes physically located within the same NAS or Expansion Unit, rather than creating volumes across your NAS and Expansion Unit.”
I’m considering getting a dx517 as well so this was a nice rabbit hole to go down. I think your gut feel about the single esata cable is accurate.
Edit: And drive order shouldn’t matter. Obviously take a backup if you can but you should be able to power down, move the drives, and power back up and it continue to work without skipping a beat. At least using the default SHA RAID or w/e.
I’ve never done it but it is my understanding that you can move the drives around. The expansion unit literally just makes it seem like you have more slots in your existing system. The system identifies the drives by their UUIDs which is why I’m thinking it isn’t a crazy idea to be able to move them around.
I had a nas server with a raid card where I had to ensure a specific drive went to a specific sas port but I think this is more like ZFS where you (and in this case DSM automatically) can just identify drives by their ID.
I like the idea of a second unit but in my case the newer models don’t seem to use intel anymore and don’t have h.265 decoding offload. If that isn’t important to you or you’d buy an older model then maybe not a terrible way to go.
Finally, I don’t know your comfort level but building a little NAS server from older gen server/workstation parts off of ebay (minus the drives) will get you pretty far bang-for-your-buck wise.