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Early 2011 model, A1278 / 2.3 GHz i5 or 2.7 GHz i7 processor.

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SSD slows when changing Bay (between main bay and optical bay)

SSD running OS X [Sierra or High Sierra] can be made to perform well [= read/writes at 400-500 MB/s ] in both Sata III bays (whether the main bay or the optical drive bay).

Problem: When moving SSD from one bay (main bay or optical drive bay) to the other, it would perform very slowly. The SSD seems to work fine in whichever bay the SSD was in when it was originally installed (whether the main bay or the optical drive bay). It was only when the SSD was moved to the other bay that I began to see highly variable read/write times, which at times were normal [400-500 MB/s ], but most times were very [<100 MB/sec]. Running Sierra and High Sierra (problem manifests in both versions of OS X).

Note 1: I have tested both my main bay and my optical drive bay, and both are Sata III (I have achieved read/write speeds between 400-500 MB/s on both bays).

Note 2: The problem is not SSD specific. For example, I have built OS X Sierra on a Samsung 850 (500 Gb) in the optical drive which functioned fine [400-500 MB/s ] until it was moved to the main bay, it became too slow to be useful. When I reinstalled OS X Sierra on the same drive using the Sata connector in the Optical drive if functioned fine [400-500 MB/s ], until I moved it to the main bay, at which point it became extremely slow. I have repeated the same experiment with a Crucial MX 500 to the same effect.

Note 3: Seem to have the same problem whether in Sierra or High Sierra

Any Ideas?

Отвечено! Посмотреть ответ У меня та же проблема

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1) Main Drive: I upgraded SATA cable for Main Drive Bay (2012 cable). All SSDs now function properly in this drive bay. This problem is resolved. 2012 cable is much thicker than the original 2011 cable, which was clearly the cause of the problems with this bay. (Not sure why the Samsung 850 worked well with the old cable for 18 months and then stopped; in any case, the new cable fixed the problem.)

2) Optical Drive: the Samsung 850 still works at full speed in both bays (optical drive and main bay). The Crucial MX500 does not. If OS X is installed on SSD while in the main bay, it works at full speed in the main bay but not the optical bay. Not sure why, and I no longer care enough to continue troubleshooting (since as Dan pointed out that this bay can’t properly support an SSD in sleep mode well enough to use this bay for the Operating system).

Thanks for all your help, the comments were important to helping me resolve the problems with the main bay.


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You’ve hit a known issue within some MacBook Pro systems!

Review the Red notes here OWC - Data Doubler The issue is the system not the carrier. The Optical drive bay is just not able to run at SATA III (6.0 Gb'/s)

To add to the problem your Samsung SSD being an auto setting drive (one that is able to match up with the systems SATA ports data rate) is unable to sync up with the port so it makes this worse. You’ll need to get a fixed SATA II (3.0 Gb/s) drive to use your optical drive port like OWC - Mercury Extreme SSD.

Don’t even think about swapping your HDD drive to the optical bay, as you’ll be only putting it at risk! The HDD bay has special crash guard services to protect your drive when you bang your system or put your system into sleep mode and move your system about.

Update (09/16/2018)

OK, if your system is not marked with a Red note then you do have a system which is able to support SATA III I/O (6.0 Gb/s) don't trust the hardware profile. Just because the PCH offers SATA III (6.0 Gb/s) the drive link speed is what you need focus on with the Optical bay drive. If it does not link up at SATA III (6.0 Gb/s) then you still have an issue with the bay)

As far as the HD bay you likely have the original SATA cable which was only rates for SATA II (3.0 Gb/s). You'll need to get the newer version of the cable MacBook Pro 13" Unibody (Mid 2012) Hard Drive Cable This cable is the better one the (and yes! its for the ‘12 model). You also want to place electricians tape on the uppercase where the cable crosses over as the rough surface of the aluminum case often kills them. Lastly, you’ll want the arc the corners don’t bend them into tight creases! The foil wires inside will be damaged!

MacBook Pro 13" Unibody (Mid 2012) Hard Drive Cable Изображение


MacBook Pro 13" Unibody (Mid 2012) Hard Drive Cable


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Thanks Dan,

Quick response appreciated. I forgot that these old machines cut power to Optical bay when in sleep mode. Indeed, when connected to the optical bay, my system cannot recover from sleeping. So, not a solution for the OS.

Per the OWC report, some 2011 MacBooks have Sata III optical bays, as seems the case with mine. [How else explain how read/write speeds of (400-500 MB/s)? … much faster than the 3Gb/s Sata II limits.

The problem remains the inability to swap drives. The OS X was installed on an SSD the optical drive, and for some reason I can't just pop that SSD into the main Sata bay without the speeds dropping off.

As stated above, its likely NOT a hardware issue, since just last week, the same SSD was booting and running OS X at full Sata III speeds (400-500 MB/s) in the main bay and since I have another SSD that and runs just fine (350MB/s-450MB/s) in the main bay (but too slowly in the optical bay). The problem seems to be the swap itself.

Any ideas ?



I appreciate the info about the 2012 drive cable being different. I had read many posts about people replacing the drive cable to fix their problem, but none had specified that you have to upgrade to a 2012 version of the cable rather than just replacing the cable (with the 2011 version). That info was key; I've bought the new cable and upgraded and it solved all the problems with the main drive bay.

Thanks a million!



Don't forget to accept the answer as your answer. Thanks!


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EDIT: Sorry, Posted this a while ago but got marked as spam.

Pretty much what Dan said…

Not entirely sure, but I remember reading that Apple did not recommend using a 6gbps capable SSD in the optical bay, but to only use 3gbps.

That may, or may not, have something to do with it.


”Testing has demonstrated that Apple factory hardware does not reliably support a 6G (6Gb/s) Solid State Drive or Hard Disk Drive in the optical bay of 2011 and 2012 MacBook Pros (Model ID 8,1; 8,2; 8,3; 9,1; 9,2). “

from https://eshop.macsales.com/item/OWC/DDAM...

Otherwise, make sure trim is enabled and run fsck -fy on the drive in question.

Apparently that has helped some people resolve speed issues.

and of course, the usual reset PRAM and SMC… :)

Hope this is of some help.

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Thanks for the info. I will go back to an HDD or DVD in the optical bay, other than temporarily using a SSD in the optical bay when doing maintenance.


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D. K. будет вечно благодарен.
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