OWC Helios & Accelsior SSD: The Best of Thunderbolt's Potential

When Intel introduced the world to Thunderbolt (then known as Light Peak) at the 2009 Intel Developer Forum, the industry was immediately intrigued by the possibilities afforded by the high speed and versatile interface. The ability to combine multiple protocols, including video, audio, data, and networking, over a single, ultra-fast cable had the potential to revolutionize the way we think about computers, peripherals, and connectivity.

After a few more years of development and refinement, Thunderbolt finally launched in early 2011 exclusively on Apple’s MacBook Pro. The interface was later expanded to the rest of Apple’s computer lineup (with the exception of the tragically neglected Mac Pro), and to Windows-based PCs on some new motherboards.

Initially, a few external hard drives were the only Thunderbolt devices available for consumers. Apple expanded the Thunderbolt device market with the Thunderbolt Display in mid–2011, but there continued to be a noticeable dearth of exciting options. That is, of course, until recently.

Other World Computing (OWC), the long-time Mac hardware and accessory company known for creating unique products, released the Helios PCI Express Thunderbolt Expansion Chassis earlier this year. The device allows users to connect a compatible PCI Express card to their Mac via Thunderbolt, opening up a world of heretofore impossible expansion options and finally fulfilling the original promise and ambition of the Thunderbolt interface.

Technical Specifications & Design

OWC Helios Review

The Helios accepts one half-length, full height, single-width PCIe 2.0 card at up to 8x speed. Not every PCIe card is compatible, however, and those getting excited at the possibility of attaching an external GPU via the Helios will be disappointed, as high-powered cards such as PC GPUs are not supported (not to mention a lack of driver support in OS X). However, the list of compatible cards – including options for external SSDs, RAID controllers, Ethernet adapters, audio and video capture and playback cards, and FireWire or eSATA adapters – is quite extensive.

OWC Helios Review

The rear of the Helios contains connections for the power adapter and two Thunderbolt ports, a simple yet powerful layout. The second Thunderbolt port enables the Helios to be part of a multi-device Thunderbolt chain, something that sets Thunderbolt apart from other high-speed interfaces such as USB 3.0.

There are no physical buttons on the unit, as the Helios intelligently powers itself on and off based on the power status of the connected Mac. Drivers for the Helios itself are also not needed, and any version of OS X later than 10.6.8 can see and communicate with the device out of the box. While the Helios itself requires no drivers, certain PCIe cards might, so users should be sure to check for necessary drivers before installing a new PCIe card into the Helios.

The Helios enclosure is small, about the length and width of an iPad mini, but not exactly portable. The smooth black aluminum device is clearly meant to find a semi-permanent home on your desk or workstation. A blue light shines on the front of the device to indicate that it has power, but there are no other flashy and distracting indicators.

Another sign that the Helios is powered on is the built-in cooling fan, which turns on and off automatically with the device. The fan is not loud, but it is certainly noticeable, especially next to a whisper-quiet iMac. In rooms with other external devices, such as our test area, the noise of the fan is quickly drowned out by the overall hum that tech enthusiasts have come to expect from their work areas. For users planning to add a Helios to a near-silent workstation, however, it should be expected that the noise of the fan will be noticeable. Thankfully, even in that scenario, the pitch of the noise is low and steady and likely to be overlooked after a few minutes of operation.

In short, the Helios looked great on our desk next to our test iMac, and it was a bit sad to pack it up and send it back to OWC at the end of the review period.

Expansion Options & Benchmarks

As mentioned above, there are many compatible PCIe card options for the Helios, but OWC’s flagship combination is a pairing with its Accelsior PCIe SSD. Utilizing two MacBook Air style SSD blades in a RAID 0 configuration, the Accelsior is a blazing fast SSD option that, until now, was only compatible with PCs and the Mac Pro. TMO’s John Martellaro took a look at the Accelsior in a Mac Pro installation earlier this year, giving the device a rating of 4.5 out 5.

OWC sent us their 480 GB Accelsior to test with the Helios, and we eagerly installed the card in the Helios enclosure. Opening the Helios and installing or removing PCIe cards is simple. A few screws on the bottom and rear of the device hold the outer shell in place. Once removed, the shell slides off, revealing the PCIe card, slot, and circuitry. It’s an easy and straightforward process for anyone with experience installing PCI cards in PCs or older Macs, and for those who are unfamiliar, OWC includes a detailed user guide with step-by-step instructions.

With the Accelsior SSD installed, we connected the Helios to power and to a 2011 27-inch iMac running OS X 10.8.2. Almost instantly, the Accelsior SSD mounted to the desktop and was ready for use. When OWC says that no drivers are required, they mean it.

Ready to measure the performance of the Helios, we first launched BenchTest, a benchmarking tool that’s part of the Drive Genius disk monitoring software. BenchTest measures random and sustained read and write performance at a variety of sample sizes.

OWC Helios Accelsior Review

The sheer performance of the Helios combined with the Accelsior SSD is immediately apparent. Reads, both random and sequential, topped out around 525 MB/s, while writes scored about 450 MB/s, both highly respectable numbers. Wanting to take another look at raw transfer speeds using a different tool, we switched to DiskTester, part of the diglloydTools package from Lloyd Chambers, which measures sequential read and write performance with different sample sizes.

OWC Helios Accelsior Review

With DiskTester, we see nearly equivalent performance, with reads topping off around 540 MB/s and writes between 400 and 425 MB/s. So far, so good. We next wanted to look at a simulation of real-world performance, so we turned to Blackmagic’s Disk Speed Test, which measures raw transfer speed to gauge a drive’s capability to read and write various video streams.

OWC Helios Accelsior Review

Using a sample transfer size of 5 GB, we measured the average peak speed achieved by the drive and were happy to see nearly 700 MB/s of read performance and about 530 MB/s in writes. The roughly 700 MB/s of sustained reads for large files easily beats the fastest currently available single SSD configurations and all but the most costly and high performance RAID configurations.

Another application that attempts to test real-world performance is AJA System Test. Like Blackmagic Disk Speed Test, AJA System Test simulates the transfer of various video sizes.

OWC Helios Accelsior Review

The test reveals equally impressive performance, with over 700 MB/s writes and 675 MB/s reads with the largest and most sequential 4K video test file.

Accelsior Windows Performance

The OWC Accelsior SSD is a versatile device, and as such fully supports Windows platforms as well. To gauge the performance of the drive, we removed it from the Helios and installed it into a x16 PCIe slot on a computer running an ASUS P8Z77-V motherboard, an Intel Core i7 3770K CPU, 16 GB of RAM, and Windows 8 Pro.

The card is fully bootable on both Mac and PC platforms, but to test the absolute best performance, we installed the Accelsior as a secondary drive and booted to an internal 2.5-inch Samsung 840 Pro SSD. We then ran ATTO Disk Benchmark, which measures sequential performance at a variety of sample sizes.

OWC Helios Accelsior Review

The direct connection to an x16 PCIe slot and the lack of Thunderbolt as the “middle man” allowed us to truly max out the performance of the Accelsior, reaching peak sustained read and write speeds of about 760 MB/s. So Windows users take note: the Accelsior SSD is a viable option for those seeking a PCIe-based SSD, either as a bootable system drive or as an ultrafast scratch disk.

Alternatives for Speed

The Helios coupled with the Accelsior SSD is certainly fast, but it is not necessarily the fastest option for your Mac. If your Mac can support two SATA III drives, and you’re looking for the absolute best performance, a dual-disk RAID 0 SSD configuration may be the way to go.

Our test iMac has two 240 GB OWC Mercury Extreme Pro 6G SSDs installed. We configured the drives in a RAID 0 volume and ran some comparisons between the Helios/Accelsior combo.

OWC Helios Accelsior Review

OWC Helios Accelsior Review

OWC Helios Accelsior Review

OWC Helios Accelsior Review

The combination of the two internal SSDs in RAID 0 clearly outperforms the Helios and Accelsior, reaching well over 800 MB/s. The internal option is also cheaper, assuming you’re comfortable installing your own hard drives. Two 240 GB SSDs of the same make as our test system currently run about US$590. The Helios with the 480 GB Accelsior is priced at $1,080.

Therefore, if pure speed is your primary concern, and you plan to use the Helios only with a PCIe SSD, going with an internal RAID array may be a viable option. But there’s far more to the Helios than just storage.

Other Uses

As we mentioned at the beginning of the review, the Helios has an extensive list of compatible PCIe cards. Everything from professional audio and video capture cards, to FireWire or USB 3.0 expansion cards, to networking adapters are now available to any Mac with a Thunderbolt port.

We tested compatibility by plugging in a Blackmagic Intensity Pro, a small video capture and playback card that supports analog and digital input. Using Blackmagic’s capture software and an external HDMI video source, the system saw the card immediately as if it were installed in an internal PCIe port on a Mac Pro. We successfully captured video and noticed absolutely no difference between using the card via the Helios and our former Mac Pro test bench.

With eSATA and USB 3.0 expansion cards available, it’s also possible to add those ports to 2011-era Macs, which fell one generation shy of USB 3.0 support. Or consider adding a 10 Gigabit network adapter for connecting to fast enterprise networks with any recent Mac.

It’s important to note, however, that the compatibility list is truly inclusive, and users should not expect PCIe devices that are not on that list to work properly or at all. This limitation was underscored when we could not get an OCZ RevoDrive PCIe SSD to work in any configuration, and an older eSATA card absent from the list also failed to work properly, although we could at least see the attached drives.

It are these other uses for the Helios that help justify the expensive price mentioned above. Users who only need a fast external SSD currently, but who may find use for other PCIe configurations down the road, will want to strongly consider investing in the Helios. A bit more money up front enables significant flexibility later.

Also, those professional users who rely on PCIe-based interfaces for audio, video, or data management will find the Helios liberating. These users, who previously were limited to using the aging Mac Pro and its PCIe slots, can now pick up a new iMac, Mac mini, or MacBook and still have full access to their compatible PCIe devices.


As fast and appealing as it is, users of the Accelsior SSD, either with the Helios or directly connected to a Mac Pro or PC, must take note: as we’ve mentioned throughout this review, the Accelsior SSD uses two blade SSDs in a RAID 0 configuration. In such a configuration, if either drive were to fail or become corrupted, all data on the drive would be lost. There’s a reason why RAID 0 is commonly known as “scary RAID.”

While such a reality should not stop someone from using the Accelsior, proper data security measures should be taken. All critical data stored on the drive should have at least one, and preferably two, backups on other storage media. Although the risk of a drive failure is relatively low, if anything should indeed happen, all of your data will be safe elsewhere, and you’ll be free to rebuild the Accelsior SSD or, if necessary, take advantage of OWC’s excellent customer service to obtain a replacement under the three year warranty.


The Helios is one of the first products to truly realize the promise of Thunderbolt. Its sleek and attractive design can be configured to house any number of PCIe cards, enabling access to previously unavailable functionality on any Thunderbolt-enabled Mac.

When coupled with the Accelsior SSD, the pair perform quite well, although not as fast as two internal SATA III SSDs. Still, for Macs without multiple drives, such as the new line of MacBook Pros and the last two generations of MacBook Air, or for those who wish to preserve their internal storage, the Helios and Accelsior offer a speedy option.

The noticeable fan noise of the unit may bother some, but it is so low that it will likely be drowned out by the hum of any other actively cooled electronic device in a given setup. Additionally, the steep price will keep the Helios and Accelsior out of the hands of most consumers. But this is truly a professional product, and for those who need the capabilities that the Helios brings, the benefit of the unit is priceless.

Product: Mercury Helios PCIe Thunderbolt Expansion Chassis

Company: Other World Computing

List Price: US$349.00 ($1079.99 w/480 GB Accelsior)



Attractive design, no drivers required for Helios (some drivers required for certain PCIe cards), extremely fast performance, bootable, versatile.


Expensive (especially when coupled with Accelsior SSD), noticeable fan noise, limited compatibility with PCIe cards.