I'm planning to build a very cost-efficient NAS around a mini-ITX motherboard using a case and an 430W PSU I have laying around. The motherboard I'm looking at is very cheap, but provides only 2 SATA ports. Since the PSU has no problem handling additional 2-3 HDD's, I was wondering if I could use the USB ports on the motherboard for hard drives mounted inside the case. I am aware of the limitations in transfer speeds, but the aim of the project is long-term archiving, so I'm completely okay even with USB 2.0 speeds.
My problem is that I can't find cables that would let me power the drives from the PSU while connecting them to a USB port. Everything I saw connects to both the SATA and the power port of the HDD's, and that would require additional power adapters, rendering the whole idea of repurposing the case and PSU impractical. So is this just a design-thing, or the drives need to get their connections from the same source?
I'm looking forward to your answers both regarding the tech behind this, and possible solutions for the problem.
Cheers!
Edit: I am planning to use 3.5" drives, so using only USB power is not really an option.