Explaining the Mac Studio’s removable SSDs, and why you can’t simply swap them out

Apple's new Mac Studio desktop began arriving in customers' hands last week, and some of those customers wasted no time in taking the machine apart. Among the more interesting discoveries was the sheer size of the M1 Ultra and its voltage regulator modules (VRMs); in addition, it seems that the Studio includes removable storage rather than the soldered-down NAND chips that most Macs use. In theory, this could make the Mac Studio the first new Mac (outside of the Mac Pro) to support upgradeable storage in quite a while.

Because the Studio's SSD slots aren't compatible with regular M.2 SSD sticks that you might use in a PC, YouTuber Luke Miani decided to test the Studio's removable storage by swapping storage from one Studio into another. He found that, while the drives are physically swappable, his Mac Studio wouldn't boot after the fact—the desktop's power LED would only flash an amber-colored "SOS" pattern. This persisted both when he tried to install the second SSD module in the Studio's second storage slot and when he tried to install an SSD from one Studio into the other Studio's main SSD slot.

"What Apple is doing here with the Mac Studio is simply inexcusable," Miani concluded. "Apple does not care about your right to repair, make no mistake. What we've seen here today is that Apple is intentionally, deliberately restricting your access to your own device. In my opinion, this is actually worse than soldering the storage onto a logic board."

Deeply sympathetic as I am to the goals of the right-to-repair movement, and deeply frustrated as I am by Apple's storage prices relative to other high-end SSDs, Miani's conclusions are based on incorrect assumptions about how modern Mac SSDs work. It's also likely that these modular SSD slots actually do facilitate easier upgrades and repairs than, say, desoldering NAND chips from a logic board and soldering on higher-capacity NAND chips. There are just caveats you need to be aware of first.

Three incorrect assumptions will be explained here, and we'll take them one at a time:

• Because the Mac Studio has physically removable SSDs, removing and replacing them should work as it does in a PC.
• Apple implemented some kind of "software block" to prevent the Mac Studio from booting after its storage has been replaced, as evidenced by the power LED's blinking amber "SOS" pattern.
• One of the Mac Studio's two SSD slots is inoperative in some configurations because of a missing SSD controller.

To address these three issues, I've pulled together Apple's documentation about how its chips work, as well as information from a Twitter thread by developer Hector Martin. He's part of a team that has been working on Asahi Linux, the first Linux distro that runs on Apple Silicon Macs, and this work informs his understanding of how storage is handled in modern Mac hardware.

How modern Mac SSDs work

To dramatically oversimplify, all SSDs need at least two things: NAND flash chips that store data and an SSD controller that handles the particulars of reading from and writing to those chips. (Some SSDs also use a small amount of DRAM as a cache, though budget-priced and mainstream SSDs increasingly just steal a small chunk of your system's memory to perform the same operations with a minor performance penalty.)

PC SSDs like Samsung's 980 Pro or Western Digital's WD Blue SN570 all include the controller and the NAND, which is what makes them easy to replace. Each SSD is a self-contained device, usable in any PC that has a physical SATA port or M.2 slot and that supports the SATA/NVMe storage specs.

Apple's SSDs used to work this way, but starting with the Apple T2 chip and continuing into the Apple Silicon era, Apple began building storage controllers directly into its own chips instead. This means that the Mac Studio's SSD cards, while removable instead of soldered down, are just NAND plus what Martin calls a "raw NAND controller/bridge." They aren't self-contained SSDs that can be swapped in and out at will, as they can on a PC. They are NAND chips that are read from and written to by the T2 or M1's built-in controller.

Martin speculates that if you use both SSD slots in the Studio, the NAND modules "definitely need to be the same size, and they might need to [use NAND chips from] the same vendor." In other words, the SSD controller built into the M1 is designed to work with specific NAND modules in specific configurations. Mixing and matching, as Miani tries to do, might fail because of mismatched NAND, mismatched capacities, or both.

It could also be that Apple doesn't support the use of a 1TB NAND module in each of the Studio's SSD slots—which is what Miani tried to install— because it isn't a configuration that Apple ships. But this is just a guess, since I'm not sure if a 2TB Mac Studio uses a single 2TB NAND module or a pair of 1TB NAND modules or if Apple's SSD controller cares how large the NAND modules are, so long as they're a matched pair and the system has been reset properly (more on that below).