OWC’s Mercury Accelsior SSD: Blistering Fast

The movement to Solid State Drives (SSD) in our Macs is now in full force. Other World Computing has introduced an SSD card designed for the PCIe slot of a Mac Pro or an Xserve that is plug-and-play and really fast. Riduculously fast.



The Apple Mac Pro and the Xserve have been used by professionals for extreme computing: advanced computation, simulations, video processing and imaging. While the Xserve is no longer being shipped by Apple, it would be really nice to exploit the PCIe slots of the Mac Pro for an ultra-fast SSD drive, and Other World Computing has delivered one in multiple sizes: 120, 240, 480 and 960 GB. To add to the amazement, it uses a RAID 0 stripped configuration for even more speed.

The reason for the use of the PCI Express slot instead of one of the drive bays is that the SATA ports in the Mac Pros are limited to 300 Megabytes/sec (MB/sec) throughput. However, the PCIe slot can handle up to 1 GB/sec. (Roughly the same as Thunderbolt’s maximum of 1.25 GB/sec.)


Image Credit: Other World Computing

However, the OWC Mercury Accelsior isn’t just any SSD. It’s basically two SSD drives stripped together, with a Marvel RAID controller, in a RAID 0 mode. That results in a performance in the 820+ MB/sec range for read and 760+ MB/sec for write, claimed by OWC. Compare that to a standard SATA 2 hard disk drive that’s in the 100-200 MB/sec range or a typical SATA 2 SSD that can deliver, roughly, 250-400 MB/sec.

Executive Summary

This product is a low profile PCI Express card that’s very easy to install in a Mac Pro, is pre-formatted for a Mac, and was tested to obtain a least 600 Megabytes (that’s bytes) per second of throughput for sustained reads or writes. You can install OS X just like any HDD and boot from it. A 480 GB storage version sells for $800 and has a three year warranty. Read on for the technical details and benchmarking.


The Mercury Accelsior is compatible with the following Macs and Xserves.

  • Mac Pro, mid 2010 (5,1)
  • Mac Pro, early 2009 (4,1)
  • Mac Pro, early 2008 (3,1)
  • Mac Pro, 2007 (2,1)
  • Mac Pro, 2006 (1,1)
  • Xserve, early 2009 (3,1)
  • Xserve, early 2008 (2,1)
  • Xserve, late 2006 (1,1)


Mac Pro

However, there are some technical details that customers should be aware of regarding the various models and slot specifications. OWC explains:

“For full throughput on Mac Pros:

  • 2009-2010 Installing Accelsior into any open x4 or x16 PCIe 2.0 slot will deliver full throughput up to 785 MB/sec.
  • 2008 Mac Pros have two x16 PCIe 2.0 slots, which enable Acceslsior to deliver full throughput up to 785 MB/sec. Installing Accelsior into of the two x4 PCIe 1.0 slots will result in Accelsior being addressed as a gen one, one-lane card with performance limited to real world 190-200 MB/sec data rates.
  • 2006-2007 Mac Pros utilize PCIe 1.0 slot which cannot be configured (even with the Expansion Utility in OS X) to address Accelsior as anything but as a first generation one-lane card. As a result, Accelsior performance will be limited to 190-200 MB/sec data rates. If maximum data rate speed up to 285 MB/sec is desired, we recommend the installation of a 2.5 inch OWC Mercury SSD in an open Mac Pro drive bay.”

Physical & Electrical Design

The Mercury Accelsior is a single, low profile PCI Express card approximately 7.1 x 4.74 x 0.81-inches (180 x 120 x 21 mm). It uses dual LSI Sandforce SF-2281 series SSD processors and a Marvel RAID 0 controller. The 480 GB unit I tested has a listed formatted capacity of 447 GB (using the old method). Starting with Snow Leopard. using base 10, one sees about 479 GB available. Of note is the listed operating temperature range of from 32F to 158F. (It can get warm inside the Mac Pro, even with multiple fans.) The non-operating (storage) temperature limit is over 300F.

The full specifications are found on OWC’s product page.

Unlike a standard SSD, like the ones found in Apple’s iMacs and MacBooks, this device, operating as a RAID system, does not support the TRIM function. Nor does it need to, according to the product manager who directed me to this blog entry at OWC.

Feature List

OWC lists the following features of the Mercury Accelsior PCI Express SSD.

  • Plug and play installation. No Drivers required
  • Out of the box, formatted as HFS+ and GUID partition
  • The only Mac bootable PCIe SSD card on the market
  • Sustained transfer rates greater than 750 megabytes/sec
  • Intelligent block management and wear leveling
  • Error correction and corruption prevention
  • Slight over provisioning (7%) allocated for data management and to maintain the SSD’s high-performance level.
  • Up to 960 GB capacity
  • Three year warranty

Mac OS X 10.6 or later is required. This product can also be used with PCs (Vista or later) and Linux (Ubuntu Linux 9.04 and later or Fedora Linux 14 and later.)


I installed the unit in my “early 2009” Mac Pro, 2.66 GHz, quad core, in the x16 slot. This is slot #2. (Slot 1 has a NVIDIA GeForce GT 120 GPU.) According to the Mac Pro notes above, I could have installed it in one of the x4 slots, that is #3 or #4, but elected not to because an early version of the Quickstart guide was slightly out of date and steered me towards the x16 slot. I could have used any slot in this model.

Inside 2009 Mac ProInside the 2009 Mac Pro (Image Credit: Apple)

Installation was very easy and fast. I released the knurled knobs that hold the restraining bar for the slot covers, removed it, then removed number 2 slot cover, and dropped in the Accelsior. (Of course, I was well grounded to avoid any static build-up.) On my Mac Pro, there’s a thin, cylindrical bar that the card rests on. The card doesn’t clip in, but rather just rests on the bar to keep it away from the motherboard and remain level. See photo below.

Accelsior Install

Installation detail.  Image Credit: author

I secured the restraining bar with the knurled knobs, closed up the Mac Pro and rebooted. It was a simple affair. As OWC says, if you can install memory, you can install this card. (It’s probably even easier.) I should note that there is an extra part in the package, a PCI slot attachment plate. It’s for PC/low profile server applications, not used for the Mac Pro.

All cozy

All cozy in the x16 slot, #2. Image Credit: Author

The first thing I did was take a look at Disk Utility in order to verify that the the Mac saw it properly. It was exactly what I wanted to see, a formatted drive up and running.

Disk Utility

Install. Boot. Bliss.

Note that, unlike a conventional drive, this device does not support S.M.A.R.T status monitoring (because it utilizes RAID).

The next thing I did was to install OS X 10.7.4 “Lion” and prepare for some benchmark testing and boot timings. All told, this was all very easy to do. So long as you’re well grounded during the install and are comfortable opening your Mac Pro, as Mac Pro users are inclined to be, then things will go very nicely.


Included in the box is a one page Quickstart guide that reiterates the system requirements, warranty information, technical support contact information and the hardware installation.

I noted that in “Step 5,” there is direction to use the Driver & Software Installation disc “for your computer’s operating system.” That should be clarified to mention that no driver is necessary for a Mac.

There really is no need for anything in addition to the Quickstart guide. After all, once a drive is working, there’s nothing else to know or do — besides peruse the specs on the product page, then get to work.


1. Lion boot

My boot drive is the factory 640 GB drive from Western Digital, model WD6400AAKS-41H280Media. The boot time, from beep to login screen was 57 seconds. With the same version of Lion (but a clean install) on the OWC Accelsior, the boot time was 22 seconds.

2. Drive Genius 3.1. Drive Genius from Prosoft Engineering has a very nice, graphical test mode. Here are the three items tested and compared.

  • Drive Genius built-in comparison: Intel Mac Pro, Early 2009, 2.66 GHz. Shown in blue as a baseline. Basically the same as my own factory drive.
  • An external Mercury Elite Pro, FireWire 800 drive. Green line on chart #3.
  • The OWC Accelsior, 480 GB. Green line on charts #1,2.

Note that, as is customary, the horizontal axis is the block size transferred.

Sustained read

Chart #1: Sustained Read. Max. value for SSD: 610 MB/sec.


Sustained Write

Chart #2: Sustained Write. Max. value for SSD: 607 MB/sec.

I’ve shown the most representative, interesting comparison between the Mac Pro factory drive (in blue) and the Accelsior (in green) without going overboard with charts.  The idea is to show clearly and quickly what this product can do.  However, I’ll throw in one more, just for comparison, because I was curious.  I have an OWC Mercury Elite Pro, 2 TB, external FireWire HDD used for the Time machine backup.  Here’s how it compared to the internal drive. I’ve selected just the write speed for brevity. And note the vertical scale change. It gives you an idea of the scale of performance across those three technologies. (If you’re curious about Thunderbolt, that’s another story and another review.)


FireWire 800

Chart #3: For comparison: an external FireWire 800 drive (green) compared to factory, internal HDD.


Now benchmarking can be a very difficult science. It can depend on factors that are often hard to identify and account for, and there can be variations between software tools. For example, I have invested only in Drive Genius, but OWC uses SpeedTools, diglloydTools Tools, and AJA System Test, according to the product manager. One can expect variations.

After I did the boot time testing, I was careful to make sure not much else in the way of user processes were running, but background processes are much harder to control.

In any case, a result like the above isn’t unusual. My testing was informal while OWC’s in-house testing is probably much more controlled and used different benchmark tools. The take away is more important, and that’s that this “drive” is really, really fast. And it’s not hard to get 80 percent of the manufacturer’s tested maximum with very little effort.


The product was released on May 12, 2012 and the 480 GB model that I tested had an initial price of US$919.99. The price has come down nicely since then. All sizes are available for immediate delivery. OWC’s current prices are:

  • 120 GB - $337.99
  • 240 GB - $499.99
  • 480 GB - $799.99
  • 960 GB - $1619.99


The product is well protected and packaged. Image Credit: the author


The OWC Mercury Accelsior PCIe SSD RAID 0 offers some serious advantages, despite the high price compared to a conventional hard disk. One should carefully weigh, these days, the low cost of an HDD in light of its so-so performance, spin-up lag and customary headaches.

First, of course, is that there are no moving parts to wear out in an SSD. Second, the quietness and quickness completely change the character of a Mac. This reviewer has vowed that he’ll never again purchase a Mac with an HDD as the boot drive. It changes your Mac experience completely, and I have two years experience with an iMac (256 GB Apple SSD) to back that up. Here’s what happens. You blink, at first, at the price.  Then you use a Mac with an SSD for a few days, and you’re converted. Permanently. You never want to go back. Ever.

Finally, using the PCIe slot leaves an extra drive bay open if one wants to optimize operational speed (SSD) with lots of cheap rotating storage in the drive bays of a Mac Pro. Alternatively, one one could install one of these devices in each of the three remaining PCIe slots for some serious corporate or scientific research horsepower. For the average user, however, it boggles the mind to think about how one could breathe new life into an older Mac Pro for merely hundreds of dollars.

I’ve been using this product since early June. It’s been flawless, and I highly recommend it.

Product: Mercury Accelsior, PCI Express SSD

Company: Other World Computing

List Price: Varies by size. See text.



Incredibly, ridiculously fast. Easy to install. No drivers required. Preformatted to HFS+/GUID partition. Three year warranty. RAID 0 performance. Technically seductive.


SSD pricing per megabyte much higher than for standard HDD.