One of the great things about the Mac Pro, and the primary reason why I’ve been anticipating it so much, is its ability to accommodate PCIe expansion. With the Mac Pro we finally have a modern Mac that can be expanded, not only via external Thunderbolt 3 peripherals, but via internal PCIe cards like the OWC Accelsior 4M2 PCIe SSD.

OWC’s newest Mac Pro-friendly accessory allows users to add a significant amount of fast solid state storage to their machines in just a few minutes. But the Accelsior 4M2 may also a viable option for MacBook Pro users looking to add external external storage to their laptops via a simple Thunderbolt 3 enclosure.


  • Installs into a full-height, half-length x8 or x16 PCIe 3.0 or 2.0 slot
  • Over 6,000MB/s real world speed in RAID 0
  • Slot-powered, no extra power required
  • Passively cooled setup
  • Configured with up to four M.2 SSDs
  • Full x4 PCIe speeds per SSD via on board PCIe switch
  • Compatible with AppleRAID and SoftRAID
  • Bootable (single disk) with macOS
  • Available now in 1, 2, 4, and 8TB configurations

Four separate M.2 SSDs

OWC provided me with a 4TB unit with a total of 3.84TB of usable storage space. The 4TB is not a single unit of flash storage, but it’s actually comprised of four 1TB M.2 SSDs in a RAID 0 configuration.

Users have the option of configuring the OWC Accelsior 4M2 via SoftRAID (preferable) or via the built-in Apple RAID Assistant. While Apple RAID Assistant is limited in the type of RAID configurations possible, SoftRAID, which OWC provides a free license for upon purchase of the Accelsior, allows for a wider range of RAID configurations, such as RAID 5.

Video review: OWC Accelsior 4M2 PCIe SSD

With all of that being said, users who purchase the Accelsior, especially the smaller storage configurations, will probably want to stick with RAID 0, as it provides the greatest amount of storage at the best performance.

Setting up a RAID 0 array using SoftRAID

Obviously RAID 0 comes with one big caveat, and that’s the fact that if one drive fails, the entire array, and all of your data, will be lost. Although flash storage failure is much less likely than it is on traditional spinning mechanical hard drives, it still remains a possibility. For that reason, I encourage you not to use a RAID 0 setup for storing irreplaceable data, and always make backups.

Accessing the drives

Although I imagine that most users who purchase the OWC Accelsior with preinstalled flash storage will simply want to install the drive and start using it, it should be noted that it’s possible to open the unit to access the flash media inside.

There are five torx screws that hold the giant heat sink on the outside of the unit securely against the four flash storage modules inside. Removing these screws and removing the heat sink will expose the flash modules, which can each be individually accessed via screwdriver.

Thus, it’s possible to upgrade the flash storage blades inside if the need ever arises in the future. And in the unlikely event that one of the blades experiences a failure, it’s good to know that you can replace them if necessary.

Installation in Mac Pro

The OWC Accelsior can be connected to either an x8 or an x16 PCIe slot inside the Mac Pro. OWC designed the Accelsior with a built in PCIe switch, which allows your Mac Pro to address each SSD at full x4 speed instead of being hard capped at x2 speeds. Each drive, when running separately via a JBOD configuration, is thus able to run at full speed.

As we noted in our initial hands-on look at the Mac Pro’s top features, installing new PCIe cards inside Apple’s desktop machine is an easy process. Simply remove the outer shell, unlock the PCIe slots, remove the slot cover, install the card, and reverse the process.

Testing the OWC Accelsior 4M2 PCIe SSD

In its marketing Materials OWC claims that the 4M2 PCIe SSD is capable of speeds up to 6000MB/s, and I was able to verify this via speed tests with AJA System Test Lite.

However, given that the Accelsior is passively cooled, it comes as no surprise that throttling occurs when the drive is pushed. This is a good thing, since heat is always a concern with a passively cooled M.2 enclosure. I pushed the Accelsior hard during one speed test for about 20 minutes, and noted that its temperature never eclipsed 60 degrees inside the Mac Pro.

But even with throttling considered, the OWC Accelsior is still faster than the 1TB build-to-order SSD option that I have inside my Mac Pro. Given the Accelsior’s speed, overall storage density, and competitive price relative to Apple prices, it’s a compelling option for Mac Pro users who require additional fast storage.

OWC Accelsior 4M2 on the go via Thunderbolt 3

One of the neat things about the Mac Pro is that it can benefit from fast internal and external storage options, thanks to PCIe and Thunderbolt 3. If you own a Thunderbolt 3 external enclosure, such as an eGPU chassis, you can use the OWC Acceslsior externally via Thunderbolt 3.

Connecting the Accelsior 4M2 to an eGPU Thunderbolt 3 chassis

Such a configuration, while lacking some of the bandwidth benefits of a direct PCIe connection, brings added flexibility to the table. It means that you can connect the drive internally or externally to your Mac Pro, but also connect externally to a MacBook Pro via Thunderbolt 3 and easily access the drive’s contents.

SSD speeds when connected to my 16-inch MacBook Pro

Thunderbolt 3 is limited to a maximum of 2800MB/s, but that’s still plenty fast for many workflows, including 4K video editing.

Booting from the OWC Accelsior 4M2

macOS Catalina doesn’t support installations on RAID-based storage, hence you’ll not be able to boot from a RAID-configured OWC Accelsior. However, if you leave one drive out of the RAID array, you can use that drive to boot, and leave the other three drives for a RAID configuration.

With a 4TB Accelsior, it means you’d have a 1TB boot drive, with a 3TB RAID 0 (or any other valid RAID config that you desire) working storage drive. That can be a pretty compelling setup for anyone who may have opted for the base model Mac Pro with its paltry 256GB SSD.

The OWC Accelsior, while not as fast as some PCIe SSD setups, is fast enough for most modern workflows. The drive is also completely silent, because it’s passively cooled, and its temperatures, even when pushed, stay within a reasonable range.

One of the unit’s most compelling features is that it comes ready to use out of the box for those choosing 1, 2, 4, or 8TB storage configurations. There’s no need to buy separate SSDs, install individual M.2 blades, etc., it’s essentially plug and play. But the drive can be upgraded, if desired, which gives you room to grow should the need arise in the future.

As I noted in our Mac Pro Top Features overview, I recommend configuring your Mac Pro with at least 1TB of on board storage, as doing so is actually cheaper than buying the 1TB OWC Accelsior after the fact. But if you need 2, 4, or 8TB of internal storage, similar to aftermarket memory upgrades, you’ll actually save money by going aftermarket, and purchasing a drive like the OWC Accelsior.

What do you think? Sound off in the comments below with your thoughts and opinions.