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Powering two disks from one OpenRD base board

The OpenRD base board is a small Marvell Kirkwood development platform. With one SATA connector, one eSATA connector and no fan, it is a tempting base for a quiet file server. The catch is that a mirrored pair of disks wants two power feeds.

The board in a nutshell

The board carries a Kirkwood system on chip, soldered memory and flash, one internal SATA port, one eSATA port and a mini PCI Express slot. It sips power and runs without a fan, which makes it a quiet base for a small server.

The problem

• There is a single SATA connector on the board.
• External USB enclosures were ruled out as unreliable for a mirror.
• An extra mains cable for a second disk was not wanted.

The eSATA port offered a way forward: it can carry the second disk's data, but the disk still needs power.

The idea

Two SATA extension cables are used, one for each disk. The power conductors of both are joined so that a single plug feeds both drives. The first disk talks over the internal SATA port, the second over the eSATA port. Both are powered from the one supply that reaches the board.

        +------------------+
        |   OpenRD board   |
        +--SATA-----eSATA-+
           |            |
        data|            |data (eSATA cable)
           |            |
        HDD1          HDD2
           \            /
            \  shared  /
             +--------+
             one power feed

Parts

• Two SATA data and power extension cables.
• One SATA to eSATA data cable.
• A soldering iron, solder and heat shrink tubing.

Method

1. Strip back the power section of both extension cables to expose the four conductors: two grounds, one five volt, one twelve volt.
2. Join matching conductors from the two cables to a single socket, keeping each wire the same colour throughout.
3. Insulate every joint with heat shrink, and make the harness mechanically secure so no joint carries the weight of a disk.
4. Label both ends before closing the case, then check continuity with a meter before connecting any disk.

Reality check

This is a modification to a power path and, if it is done badly, it can damage disks or the board. Keep the joints short, keep the current within what the supply can give, and test with a cheap drive before trusting it with anything you care about. A short harness that only ever powers disks you can afford to lose is the sensible way to experiment.

With the harness in place the board makes a tidy little mirrored file server, and the second disk finally has a job.

See also the operating system references on the resources page.