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but a separate hard drive in the main server box for backup makes sense, because most of the time the only thing that will fry is the main drive

Lol both drives would be attached to the same power supply so both would likely fry. Mechanical failure rate should be independent of the enclosure. Power supply failure would knock out both.

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I'm working on all this. Partitioning an E drive on the SSD that is only 256G to start with might not get me much. It may be time to get my IT guy involved.

That's why I suggested making the 1TB Drive as the document/user storage. SBS Essentials 60-80GB. 180-200GB for AC is quite a bit. Remember your main partition can be a lot smaller since there is no Exchange or SharePoint. It shouldn't really grow much after the initial install.