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

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New 1TB HDD won't mount internally

Hey :) I hope you can help!

I just bought a new 1TB Seagate HD as the old 500GB drive died, but I'm having trouble with it mounting internally. It didn't work so I mounted it externally in a USB enclosure no issues. I even installed El Capitan on it whilst it was externally mounted. It boots and runs fine - but I don't want to run my mac from an external drive!

When I put it back internal (into the HD bay), it gives me the question mark folder. Going into Disk Utility in Recovery shows the drive (1 TB ST1000LM024 HN...) and the volume (disk12s1), but when I click "Mount" it simply doesn't do anything.

Any ideas? Thanks for your time :)

HD SPECS: Link here.


Buffer: 8 MB

Drive Interface Standard: SATA 3

Drive Interface: SATA

Drive Type: Internal

Drive Width: 2.5"

Product Family: Momentus

Spindle Speed (rpm): 5400

Storage Capacity: 1 TB

MACBOOK: Link here.

Processor: 2.5GHz dual-core Intel Core i5 processor

Answered! View the answer I have this problem too

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Apple doesn't get into the nitty-gritty details of your system. I find the EveryMac web site as being my goto for the deeper details. As an example here is one of the systems in this series: MacBook Pro 13" 2.9 GHz i7 (Mid-2012). While you may have one of the slower CPU versions the core specs are the same.

Here is where you can verify what your HD's SATA port speed is - SATA III (6.0 Gb/s). That is not to say what the rest of the system is configured to run at. The HD's specs are what they are marked on the device (or looked up by its P/N) or what you can find in the About This Mac > System Report listing.


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You also will likely need a new HD SATA cable: MacBook Pro 13" Unibody (Mid 2012) Hard Drive Cable - Apple P/N 923-0104.

The original cable was not rated for SATA III (6.0 Gb/s) which your new HD is trying to run at. Apple used a SATA II (3.0 Gb/s) HD in this system originally which is why the new HD will work externally not internally and the if the old HD is still working will work in either case.

Here's the IFIXIT guide to replace it MacBook Pro 13" Unibody Mid 2012 Hard Drive Cable Replacement

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


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


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Thanks again Dan :) Guess I'll try get my hands a new cable then. iFixIt seem to be out of stock, and plus I'm in New Zealand anyway! Will keep this thread open till I've changed the cable and got it working.


Just the final update - it was the cable. I contacted the local Apple Certified service providers, and it turns out there's an Apple Repair Programme for this macbook. Got the cable replaced for free.

Thanks for your help everyone!


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I've had similar problems in the past - try formatting the disk (not the partition), then running first aid on the disk, then re-formatting the disk. Formatting the partition may also fix the problem.

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Thanks for your response Sam :) I've tried formatting the disk and reinstalling El Capitan already, but no luck. You think it's worth formatting again, running first aid, and re-formatting? That's just painful haha


Also you may want to check the systems firmware level if you are using an older version of OS-X. Here's the Apple T/N: About EFI and SMC firmware updates for Intel-based Mac computers.


Thanks for your reply Dan :) I did wonder if the SATA cable would need replacing.

If I get a new cable, will that support SATA III? Or can I somehow force the new HDD to transfer at SATA II rates? The HDD does say it's compatible with SATA II.

I also followed the link and updated the SMC firmware as per your suggestions. After this and also re-formatting the drive, it won't mount internally. It must be the SATA cable right?


Yes, the new version of the cable will support SATA III (6.0 Gb/s) throughput.

The newer drives have SATA I/O speed sense when they list two or more SATA I/O speeds (not all drives offer this), so they will auto-magically match the systems SATA port speed. There is nothing you can do to alter this.

And yes you need to replace the cable.


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