Help me choose a NAS for my father's business

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These Synology and Qnap models are very similar in performance. If you have an IT person available he could configure your Qnap to be safe enough for remote access. Otherwise, Synology will be a better way to go. Both Systems allow remote access. I would recommend using VPN service. This will mount your NAS on your remote computer similar as it was local drive. The speed will depend on your internet upload/download performance. If you want something faster then 100MB/s locally, you can consider 473A series with 10gbe card or DS1522+ with 10gbe card.

I hope this helps.