Chosen Solution
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. Seagate 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
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
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.
