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).

What the Mac Studio's "SOS" light actually means

From Max Tech's teardown of the Mac Studio, a shot with the internal power supply removed and both SSD slots (one populated, one not) visible along the top.

Credit: Max Tech

"And then [Apple has] the audacity to flash an amber 'SOS' signal," Miani complained in his video. "Really? They're gonna play the victim, after restricting your ability to modify a machine that you paid five thousand dollars for, that was designed with removable storage?"

When Miani took one SSD from one Mac Studio and put it in the main SSD slot in the other Mac Studio, he was greeted with an amber SOS light. Expecting the Mac to at least try to boot in this case is reasonable—but the reason why the Studio didn't do this is, again, explained by Apple's documentation. It requires no conspiracy theories about Apple intentionally bricking Macs that have been opened and operated on.

There's one other Apple Silicon Mac with a status indicator LED: the 2020 Mac mini. And buried deep within Apple's documentation is a description of what the status indicator light means: solid white when the machine is on, pulsating white when it's asleep, and flashing amber when it's in firmware recovery mode. The amber pattern—three short blinks, followed by three long blinks, followed by three short blinks—is the exact same "SOS" signal that Miani recognized after swapping out his Studio's removable storage.

Apple Silicon Macs (and T2 Macs), like iPhones and iPads, go into a special recovery mode (also called "DFU" mode) when something goes deeply wrong with a software or firmware update. When in DFU mode, the troubled Mac can be connected to a second Mac running the Apple Configurator app, which can download a restore file that fixes your Mac's firmware and recovery OS and then installs a fresh copy of macOS.

When you swap the NAND from one Mac Studio into another one, you make all the data on that NAND unreadable. This is by design. Each M1 or T2 chip has a unique hardware identifier that is used to decrypt your Mac's storage. This is part of the Secure Enclave inside modern Apple Silicon chips, and it prevents a drive from one Mac from being removed and inserted into another (or into some kind of external storage enclosure).

Without that unique hardware identifier, the data on the drive is unreadable gobbledygook. By performing a DFU restore and resetting everything, you're also re-encrypting the drive using the new Mac's M1 or T2's unique identifier, wiping the data on the drive but making it readable to the new macOS install.

This is, in fact, exactly the same procedure you need to follow to upgrade the Apple-provided storage in a 2019 Mac Pro with a T2 chip.

Since his video was released, Miani has tried and failed to perform a full DFU mode restore on the Mac Studio, but as Martin notes, DFU restores may not be working properly on the Studio yet. Because the Studio is still new, the restore image needed to resuscitate a Studio may simply be unavailable—something I've run into before with pre-release and recently released Apple hardware and software.

Update: It looks like iFixit has gotten a simple drive swap to work using a DFU restore, though they're still working on populating the second SSD slot.

Why the Mac Studio has two SSD slots

"It is bad enough that there is a second SSD slot that is non-functional," Miani continued. "That means Apple is shipping multi-thousand dollar machines where they were too cheap to even install the controllers that were necessary to make that slot work."

Because the storage controller is part of the M1 chip, blaming a missing controller for this empty second slot is incorrect. The Texas Instruments chips that Miani identifies on the Mac Studio's board are not there to be storage controllers.

Apple includes a second SSD slot because that slot is populated with a second card in the higher-capacity versions of the Mac Studio. As Martin points out, this is just like the maxed-out Apple Silicon MacBook Pros, too, though it's not as noticeable there because the storage is soldered down rather than installed in slots. If you were to open a Mac Studio with an 8TB SSD (and possibly one with a 4TB SSD or a 2TB SSD!), you would see both slots populated. (Miani doesn't say which Studio configuration he's using, though based on the prices he quotes—$4,000 and $5,000—I would assume these are M1 Ultra configurations with the base 1TB SSDs.)

The second slot in the 1TB Mac Studio is actually the opposite of cheap, since some older Macs that shipped without SSD storage didn't actually include the physical connectors for those drives. Scroll to Step 24 of this iFixit teardown of the 2012 21.5-inch iMac for one example; the traces on the logic board for the SSD slot are there, but because this Mac was ordered without one, there's no connector soldered down. The presence of the second connector in all Studio models, regardless of the amount of storage they were ordered with, suggests that it will at least be possible to upgrade your storage down the line... provided you buy a matched pair of NAND modules and reset the system properly.