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It depends of what you want as future proof (expansion capabilities). Usually home user nases come with low power cpu, a high power cpu usually is a enterprise grade nas, really costly for a home user. So having it separated makes the cpu upgrade easy but now you have 2 boxes. But if your terramaster comes with a decent cpu I don’t see any problem.
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True nas scale is really a behemoth able of almost everything. I would start with something more reduced like omv or unraid. You really don’t need the advance enterprise features of that and it will add only complexity to the setup.
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If you use NFS for exporting folders from your nas, the “computing box” will see this as a local folder, so no need to have 2 copies of the same file.
Hope it helps
The problem with such advance Sw is the overwhelming list of options and the lacks of sane defaults
It is not the same to find 10 different (and complex) solutions when you are evaluating what you can do for solving a problem. It adds more noise to the solution than anything else. And of course the minimum resources needed ;)
For the downloading I suggest you to have the download folder and the main storage both exported under the same nfs folder. Quite handy.