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Does anyone have experiencing using a dual slot external hard drive docking station with only one eSATA port on the back? Do both drives show up in the OS even though only one eSATA cable is connected? I'm curious because there's a dock I'm looking at that has 2 eSATA ports and many others that only have one. I'm thinking that the dock male connectors internally plug into "two" eSATA host and then are redirected to 1 single output but I want to be sure.

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The result is going to depend a lot on the hardware in the dock and on your motherboard. If the dock and your eSATA controller support port multipliers, you will have no issues at all.

If not, then you will not have plug & play capabilities on both drives. You couldnt add a 2nd drive to the dock later, as the OS wouldnt see it. They would both have to be there when the device is initialized. And if both were and you unmounted one, you wouldnt be able to remount it.

Im not sure, what would happen if only one side supported port multipliers...

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  • You're a bit off on two points: first the hot plugging depends on the dock, not the controller's support for pmp at all. Second, it doesn't matter whether you mount or unmount the filesystem: without pmp support only the first drive will ever show up at all, and you can unmount and remount it all you want just like with a single drive.
    – psusi
    Dec 29, 2014 at 2:59
  • its possible, ive never had the issues myself. Just posting what ive heard.
    – Keltari
    Dec 29, 2014 at 3:40
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Don't buy the Rosewill one. I tried that one first and found that whichever drive is connected first, the dock mangles its IDENTIFY_DEVICE response so that it appears to not support 6 Gbps speeds, or Native Command Queueing or SMART health reporting and has a bogus firmware revision ( the second drive works fine ). When I contacted their tech support, they claimed that they did not even make a device that supports access to both drives at once, and then that it did not have a port multiplier in it, though it most certainly does report that is a port multiplier with two drives behind it. Rosewill claims it only supports a single drive when using the eSATA connection rather than the USB3 connection so they don't seem to know their arse from a hole in the ground.

The Thermaltake BlacX Duet works great, though I don't get great performance when accessing both drives at the same time due to my controller not supporting Fis Based Switching. You can tell whether it supports this if you are running linux by finding a line like this in your dmesg or /var/log/kern.log:

ahci 0000:05:00.0: flags: 64bit ncq pm led clo pmp pio slum part

The pmp indicates port multiplier support, and fbs indicates Fis Based Switching, which gets you better and more even performance when accessing both drives at the same time. My case also has an eSATA port on the front panel that does not work properly so I have to plug it into the back panel on the motherboard.

Without port multiplier support on the controller, the second drive won't show up at all. The hot plug capability depends on support in the port multiplier in the dock. Unfortunately, the Thermaltake one does not support it ( the Rosewill one did ) so both drives have to be plugged in when you turn the dock on. If you want to add the second drive then you have to unmount and power down the dock first. You can tell in linux because you see a line like this in dmesg:

ata8.15: Port Multiplier 1.1, 0x197b:0x2352 r0, 2 ports, feat 0x0/0x0
ata8.15: Asynchronous notification not supported, hotplug won't work on fan-out ports. Use warm-plug instead.

Note here that I am only talking about hot plugging a second drive into the dock, not hot plugging the dock as a whole into the computer while it is running, which requires no special support.

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    @ChristopherBruce, posting this answer drove me to dig back into investigating the performance issue again and it seems I may have jumped to conclusions about the fis based switching and the problem may simply be that the JMicron controller that drives my eSATA ports just sucks; it appears to simply have a maximum throughput of only ~170 MB/s even to a single drive when reading entirely out of the drive cache rather than the platters, which is more like ~270 MB/s when I connect the drive to my internal controller. I'm going to order an esata adapter board for my other controllers and test.
    – psusi
    Jan 5, 2015 at 3:05
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    @ChristopherBruce, nifty.. I got an esata bracket so I can connect the dock to my internal ports and found that the JMicron controller is just slow due to it being connected at only pcie 1x gen 1 speed, and the Intel and Marvell controllers are each capable of 270 and 240 MB/s respectively. I had to patch the kernel to disable fbs on the Marvell because it didn't work at all, giving 106 MB/s sustained to each drive simultaneously.
    – psusi
    Jan 7, 2015 at 2:36

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