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Released June 2012, Model A1278. Intel processor with Turbo Boost, Up to 512 MB DDR5 Video RAM

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Unibody Laptop Dual Drive - Upgrade Kit

The specs of the caddy state that it only supports 750Gb drives

What does this spec relates to? Drive size? Drive capacity? Transfer capacity?

Please help!

I have a Samsung 2TB super slim SATA III / 6Gb transfer that I most want to put in there

Thank you!

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OK, so I still have the issue at hand and some posts at Macrumours state that there Is an issue with the transfer rates of the Optibay.

The Seagate/Samsung Spindrive M9T has a SATA III/6Gb specs that work perfectly when the drive is inside its case (outside) and connected to the MBP using an USB 3 cable (read/write); but when I take the drive into the iFixit caddy and reinstall it, the OS can read but cannot write, and of course, with that later issue, a re-format is impossible because it cannot erase and the process leaves a drive that Disk Utility can see but the Finder cannot.

Already tried to boot from a USB, in order to avoid the mapping and mounting issues to no avail. The OS simply cannot write when the drive is in the optibay

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Sadly, Your system will need a fixed SATA II (3.0 Gb/s) drive Vs the SATA III (6.0 Gb/s) drive you have {even an auto sensing drive won't work correctly} for the optical drive caddy. It makes no difference who's carrier you get they all face the same limitation as the systems Platform Controller Hub has problems depending on the clocking of the CPU on the second SATA port.


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750GB refers to drive capacity - how much data the installed drive can contain.

'Drive size' could mean the same thing as 'drive capacity' (at least, the two terms are synonymous in my mind), or 'drive size' could refer to the physical dimensions of the drive mechanism itself. Since I'm guessing that the caddy you're talking about is for mounting a 2.5" SATA hard drive or SSD in the bay where a 5.25" SATA optical drive was formerly mounted, the only physical dimension that's in question is the thickness of the drive mechanism. The HD and optical bays on a Unibody MBP are designed to hold 9.5mm drives; as long as the Samsung is no thicker than 9.5mm, it should fit.

By 'transfer capacity', I suspect you mean bus speed. In earlier MBP generations, the hard drive was on a 6 Gbps SATA III bus, while the optical drive was on a slower 3Gbps SATA II bus. This could cause problems with newer drives installed in the optical bay, because the drives couldn't always switch themselves successfully to the slower bus. This is a big issue with the 2011 Unibody MacBook Pros; you shouldn't have that problem with the Mid-2012 MBP.

Without knowing who made the caddy you're using, I'm guessing that the 750GB limit is a CYA measure from the manufacturer/retailer. It's likely that 750GB hard drives were the largest that were readily available when the caddy was designed, but they haven't made any effort to test and verify the caddy with larger drives since then. But if the caddy is rated for SATA III, I'll bet you can install the 2TB drive in it, then format and use it without issue.

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Thank you! And yes, part of the answer that I am looking for is if the caddy can accept a 2TB drive, and if it is made only for any specific bus.

Since what I get every time that I put the drive in the caddy is that "error 50" and cannot write it

The caddy was made (sold) by iFixit and it is the one that comes as a result of a search using the title of my question.


Adlerpe is right on the money here! You have the physical height issue of the drive and the SATA ports speed issue. The problem you are facing now is the HD is not running at the correct SATA ports speed. Sadly, the optical drives SATA port is in-between SATA II & III. You'll need to find a fixed SATA II drive to do a dual drive setup in your system.


MacTracker lists the optical drive bus on the Mid 2012 Unibody MBP as 6Gbps SATA III; the transitional generation between SATA II and SATA III was the Early 2011 generation. EveryMac doesn't list a bus speed for the optical bus at all. In theory, the bus will support the drive, as long as the intervening caddy is able to support SATA III. Unfortunately, since the majority of Macs with optical bays come from the ATAPI/SATA I/SATA II era, it's likely that most caddies haven't been upgraded to the faster bus, as there's comparatively little demand for it. I'd get out and shop for a caddy that specifically supports SATA III; otherwise, you may continue to have trouble with the SATA III drive.


@Adlerpe - The SATA caddy is just a pass-through design so it technically can run at any SATA speed. Yes, some are better designed than others and if the traces are not correctly laidout there can be cross talk issues at the higher speeds. But the issue here is a signaling problem some systems have due to the clocking of the controller chip for a given CPU in the system. Checkout OWC caddy they have it documented in the notes.


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I seem to be having the same problem. I have late 2011 MBP and have bought an iFixit caddy to replace the optical drive and installed a samsung 850 EVO 500gb SSD. The drive runs fine as an external drive using the USB cable, but when it is installed internally it won't allow it to be formatted. Even when it is formatted externally and then installed internally won't allow files to be written to it or install the OS. Some of the answers are saying use another cable or replace the SSD now I am reading that it is data transfer rate issue.

The SSD works fine externally. The cable worked fine when it was attached to the optical drive.

I bought this combo because the reseller assures me it works for the majority of cases issue-free. Is there a fix for this?

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Sorry no, The issue is within the MacBook Pro's logic board. OWC has a good set of notes in their product that gets into which systems won't work: OWC Data Doubler. Remember it makes no difference who's carrier you use they all face the same limitation here.

As to the Samsung drive it is a auto sense drive so it too won't work here. You need a FIXED SATA II (3.0 Gb/s) drive.

The auto sense logic gets confused as it thinks the port is SATA III (6.0 Gb/s) which is what your system report will list it as. But, it's not able to run at this speed.

You may think you can get a SATA II HD and then put the SATA III SSD where the HD is now. Sorry, thats not smart either! As only the HD port has crash guard protection. If you bounce the system while running you'll damage the HD if you have it in the optical bay.


so Dan, then would please explain why after doing what I have done it works perfectly? It works like a brand new mac - it is much much faster and it is stable and so far no issues at all. I am an artist NOT a techie - so I go on what other people suggest to me and I simply give it a go. In this case it has worked. I don't know anything about the technical details of what you speak and while I can't absolutely say WHY it works, it DOES work. This was confirmed by a conversation with a Mac repairer and he said that is the reason why they put the SSD in place of the normal HD and the normal HD in the optical caddy. So you may suggest that I am not smart - so? It works or at very least it works on my mac in the way I describe and as my mac is my professional tool that I need for my work I have no reason to bullshit and tell you something works when it doesn't. I am simply providing my input for others who might be experiencing the same issue and this might solve their problem as well. cheers


Please re-read what I stated again:

"You may think you can get a SATA II HD and then put the SATA III SSD where the HD is now. Sorry, thats not smart either! As only the HD port has crash guard protection. If you bounce the system while running you'll damage the HD if you have it in the optical bay"

All you did here was move your Samsung SSD into the HD bay which will of course work! That is what we do! But unlike you we don't use the optical carrier for the HD at all as we have learned it's not a reliable config for our field folks. I support over 400 MacBook Pro's in the field so I've seen almost every failure you could see.

Again. the HD will work in the optical bay as long as you don't move the system while it's running. So if you use your system on a desk and don't move it you're fine! Its only the people who travel and don't wait for their systems to shut down or put them in to sleep mode and then move the system you run the risk of the HD getting damaged. The Apple HD which you had in your system was likely a SATA II (3.0 Gb/s) which is why the you didn't hit SATA port issue as well here. The newer systems have a SATA III (6.0 Gb/s) HD so it would have problems then.


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Ed Oliver will be eternally grateful.
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