It was baffling, irritating, and inexplicable. My wife’s mid-2012 MacBook Pro remains in generally good condition, but she started to bump up against the storage limits of its 500GB drive, between iPhone photos and videos and her music collection. She prefers to not use iCloud for sync and storage, especially after watching me wrestle with problems over the years. (It all works fine for me now.)
A drive upgrade seemed a logical and easy course. Apple used to make it relatively easy to pop a new 2.5-inch drive, and this model wasn’t an exception. You pop a few screws on the underside, disconnect the battery (optional, but sensible), and then remove the drive interface cable and a few mounting and locking screws. Reverse the process, and you’re all set.
I’ve done this dozens of times for desktop and laptop Macs over the years. No biggie, right? Using an external drive enclosure, I’d clone her drive. But the minute I popped the new 1TB drive into the laptop and restarted, things went haywire. Sometimes, not right away; others times, immediately at boot. If it were a drive controller or cable problem, we should have seen this happen before, or at least after I returned the original drive into its slot.
I tweeted about this frustration, and my friend Alanna asked: are you sure this laptop model can accept a 1TB drive? I was sure! Or so I thought. I researched again to make sure there wasn’t a known controller flaw or limitation, and there wasn’t. (If you can find a 4TB laptop drive that’s no higher than 9.5mm, you’re good.)
But I did find a number of people discussing an issue between 2012 and 2015 related to this and similar models: a drive-interface standard mismatch, despite seeming compatibility.
##The SATA story
Until relatively recently and for many years, nearly all desktop and laptop computers used the Serial ATA (SATA) standard in version I, II, or III as the protocol for moving data to and from the computer to a disk drive. (Apple has replaced this with a PCIe bus in modern MacBook Pros.)
SATA I offered a maximum 1.5 gigabits per second (150 megabytes per second) transfer rates; II doubled to 3Gbps (300MBps); and III doubled again to 6Gbps (600MBps). Each successive standard is backwards compatible with the previous one and they all use the same physical connector type. The mid-2012 MacBook Pro supports SATA II.
However, even though the controllers in a computer are backwards compatible, allowing a SATA III controller to work with a SATA II drive, the reverse isn’t always the case. This is surprisingly difficult to discover online, probably because there was a transition period in which computers shifted to SATA III and many drives were hybrid II/III drives that auto-sensed the controller type, could be set to II or III using "jumpers" (clips on a special area of the drive), or could have drive firmware flashed to II or III compatibility.
A SATA III-only drive apparently doesn’t just not work with a SATA II controller, but tries to move data and fails intermittently. When I upgraded my wife’s computer, it wouldn’t boot into macOS, but it would boot into Recovery on the same drive, which was doubly mystifying.
After a fair amount of searching, I found several accounts on forums from people around 2012 and 2013 wrestling with this problem, and describing their solution: get a SATA II-only drive or a SATA III drive with a SATA II mode. There’s an excellent discussion in 2013 at iFixIt, which if I’d found a while ago, I would have bypassed this entire problem.
I sought out an older SATA II-only 1TB laptop drive, which was sold new in the bag, though manufactured in 2015, from a reliable third-party seller. This time, I went through the same steps to clone and swap the drive, and it worked as expected.
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