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I have 2010 MBP.

Wanted to put SSD in optical bay (and use it as main boot drive) because main bay has SMS (motion detection) that HDD would use.

The only problem is that optical bay in my computer seems to have lower speed:

Link Speed:                3 Gigabit
Negotiated Link Speed:     1.5 Gigabit

main bay:

Link Speed:                3 Gigabit
Negotiated Link Speed:     3 Gigabit

I have Samsung 840 PRO SSD - would this lower speed be significant and noticeable?

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You should go ahead and do what you first propose of the SSD in the optical and the HDD in the main.

There are two things to consider with speed, data rate and latency. Data rate is how fast the data can transfer from point A to point B. Something to keep in mind though is HDD are not that fast and may not use the full 3 Gigabits it is given. A WD green drive is made to save power and reduce heat. This means it spins slower, so it can't pick up the data as fast. As a result you are not at transfer link saturation.

The SSD does not need to spin and and only access one part of a file at a time, it has the entire drive accessible. As a result it can saturate the line.

Latency is also important, it takes time for the HDD to get to the files that are needed, for the SSD the files are already there. As a result, during boot the computer is not waiting for the next file to come around. It can go file after file after file. Reduced latency means faster speeds.

Already your first proposed setup is wining, but while doing some research, I found answered question: What is an SSD drive's negotiated link speed?. The optical bay is capable of 3 gigabits per second, however, the negotiated speed is 1.5 because the optical drive is slow and doesn't need the 3.

The optical drive negotiated for a lower speed but can handle faster on the bus. If you already have the drive one thing you can do just test it out yourself, try one configuration then another. There is a free program called Black Magic disk speed test (itunes.apple.com/us/app/blackmagic-disk-speed-test/id425264550) that measures your hard drive speed.

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  • Ok thank you, but it seems I will loose SSD performance in the optical bay which is SATA I while the SSD is SATA III capable and is operating on SATA II in main bay now. You haven't really answered my main question: if I move my SSD to optical bay, will I notice (much) lower performance with my regular work? I know have SSD in main bay and no HDD yet installed.
    – davidhq
    Commented Apr 16, 2014 at 19:21
  • Actually now I think I understand... with help of http://www.tomshardware.co.uk/answers/id-1711511/link-speed-negotiated-link-speed.html ... so my optical drive negotiated a lower speed but the interface is actually capable of the same speed as main bay in my case (3 Gigabit) - am I right? So moving SSD wouldn't take a performance hit at all...
    – davidhq
    Commented Apr 16, 2014 at 19:27
  • That is correct and what I was trying to get at in the last paragraph. The optical drive negotiated for a lower speed but can handle faster on the bus. If you already have the drive one thing you can do just test it out yourself, try one configuration then another. There is a free program called Black Magic disk speed test (itunes.apple.com/us/app/blackmagic-disk-speed-test/id425264550) that measures your hard drive speed. You can test in both configurations to see which one is the fasted. I bet they will both be very close with the SSD wining, but would like to hear what you get.
    – traisjames
    Commented Apr 16, 2014 at 20:03
  • ok.. currently I got 25 MB/s write and 180 MB/s read. When I put SSD to optical bay, I'll compare and let you know. If you want, you can edit your answer to more clearly explain what "negotiated link speed" means... because I didn't get it immediately.
    – davidhq
    Commented Apr 16, 2014 at 22:03
  • Ok. Can't wait to hear what your SSD does in your optical and in your main bays. My current MBP just got 680MB/s Write and 620MB/s read using that test.
    – traisjames
    Commented Apr 17, 2014 at 4:50
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I always recommend putting the HDD in the best shock mount and the SSD in the least best shock mount. So leave the HDD in the HDD slot.

  • Unless you know for certain that you will have substantial time where transfers above 1.5 Gbps will happen, then there's no speed to be gained by placing an SSD on a port that negotiates to 3.0 Gbps.
  • The Time you will lose with a hardware failure far outweighs any time you might make up even if you have some burst needs for data transfer that fast.

If you have specific benchmarks that need to be met, only then would I say go ahead and time things with the drives in both positions and then re-evaluate the plan. I'm sure there is a scenario where my initial advice is sub-optimal, but I haven't come across it yet.

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