CNET logo Why You Can Trust CNET

Our expert, award-winning staff selects the products we cover and rigorously researches and tests our top picks. If you buy through our links, we may get a commission. Reviews ethics statement

Promise Pegasus J4 review: An unconventional Thunderbolt storage device

The quirky four-bay Promise Pegasus J4 is very much like two dual-bay devices combined into one.

Dong Ngo SF Labs Manager, Editor / Reviews
CNET editor Dong Ngo has been involved with technology since 2000, starting with testing gadgets and writing code for CNET Labs' benchmarks. He now manages CNET San Francisco Labs, reviews 3D printers, networking/storage devices, and also writes about other topics from online security to new gadgets and how technology impacts the life of people around the world.
Dong Ngo
6 min read

If you want a Thunderbolt storage device that can house four 2.5-inch internal drives, there are now two options, the recently reviewed Drobo Mini and Promise Technology's Pegasus J4. The choice is easy, however, since the two are quite different from each other.


Promise Pegasus J4

The Good

The <b>Promise Technology Pegasus J4</b> can house four 2.5-inch internal drives and offers fast performance. It's compact and easy to use.

The Bad

The J4 doesn't support RAID 5 or USB 3.0, nor does it include a Thunderbolt cable. You have to open the casing to service its internal drives.

The Bottom Line

The Pegasus J4 is currently the only good choice on the market if you're looking for a compact four-bay Thunderbolt storage device.

While the Drobo Mini's drive-bay design is great, the Promise J4 requires you to open its chassis to install and replace its storage drives. On the other hand, the J4 provides better performance and is more affordable.

Still, lacking RAID 5, the J4 is definitely not an ideal four-bay storage solution. It doesn't come with the much-needed Thunderbolt cable, either. But, priced at $400 (no storage included) or $800 with 2TB, the J4 makes a decent investment. If you don't mind bulkier storage devices, check out these alternatives for faster performance, larger capacity, and possibly lower prices.

Design and features
The Promise J4 is about the size of a Mac Mini, and you have to undo five screws on the bottom and lift up the top part of its chassis to get to its internal drive bays. Here, in order to install or remove drives, you need to use the screwdriver again, since each drive is secured to the chassis by another screw. Taking off the top is quite easy, and installing all four drives takes just a few minutes, but still, this is a backward design. The only good thing is that you can't accidentally remove a drive, which might lead to data loss. Promise says it's contemplating a better design for the next version of the J4.

With an aluminum and plastic body, the J4 looks good. There's an array of small LED lights on the front that shows the device's status. On the back are two Thunderbolt ports, the power connector, and a power button. You need to press and hold this button for a few seconds to turn the drive off. Most of the time, turning it off is unnecessary since the device's power status is linked to that of the Mac it's connected to: it goes to sleep when the computer is in sleep mode and wakes up when the computer is awakened and so on.

On the bottom, the J4 has a user-replaceable ventilation fan. This fan is quiet, and the J4 emits almost no sound and very little heat during operation.

Despite the difference in physical size and the number of drive bays, the Pegasus J4 is the next step up from the tiny Promise J2, and is not comparable to the four-bay Pegasus R4. This is because the R4 (like the R6) is a hardware RAID storage system, while the J devices are JBOD storage systems. This means the J4 can only provide software RAID configurations that the operating system natively supports. Since the J4 works only with Macs, it offers three hard-drive configuration options: RAID 0, RAID 1, and JBOD. (To find out more about different ways to set up internal drives, check out this post.)

Generally, it's overkill to use all four drives in a RAID 1, and extremely risky in a RAID 0. You can and should set up the J4 in two separate RAID 1 or RAID 0 volumes. In fact, the best use of the J4 might be setting up two of its drives in a RAID 0 volume to store frequently accessed data and the other two in a RAID 1 volume to store important data. This is similar to using two dual-bay Thunderbolt storage devices at the same time.

The JBOD setup option, called Concatenated Disk Set in Mac OS' Disk Utility, combines all drives into one volume, but it offers no RAID-related benefits, such as improved performance or redundancy.

Promise says that when a version of the J4 becomes available for Windows computers in the future, it will offer RAID 5. For now, the J4 works only with Mac OS X 10.6.8 or later. You can daisy-chain up to six J4 units together, to increase storage space or provide backups.

Apart from installing the internal drives, which is only necessary if you get the diskless version of the J4, setting up the device involves installing its software driver from an included CD (or you can download it from Promise's Web site). After that you need to use Disk Utility to configure the internal drives, which took just a few minutes in my trials.

Our review unit came with four Toshiba 500GB internal drives, model MQ01ABD050. These are regular laptop drives that spin at 5,400rpm. You can, however, use faster drives that spin at 7,200rpm, or even solid-state drives (SSDs). (See the compatibility list.)

I tested the J4 in all three configurations, RAID 0, RAID 1, and JBOD, and it offered good performance, considering it was using slow hard drives. As with all multiple-bay Thunderbolt storage devices, I did two sets of tests. The first, called Thunderbolt vs. Internal, has the least involvement of the test machine. In the second, Thunderbolt vs. External, I tested it by copying data back and forth from the test machine. The computer used for this test is a late-2011 MacBook Pro running Mac OS 10.7 and with a SATA 3 SSD.

Thunderbolt vs. internal
This is the test in which the reviewed Thunderbolt storage device shows its performance just by itself and when working with another Thunderbolt device. Internal drives are thrown into the mix because prior to Thunderbolt, internal drives were the fastest storage devices on the market.

In this test, the J4 excelled in RAID 0, scoring about 300MBps when copying data from another Thunderbolt drive. When it was set to copy data within itself, doing both reading and writing at the same time, it registered 92MBps, about the average.

It was quite different when the J4 was set up in RAID 1, however. Now it scored 103MBps when copying data from another Thunderbolt device and just 37MBps when copying data within itself.

Finally, in JBOD, the J4 registered 103MBps and 39MBps for the unit-to-unit and self-read and write tests, respectively.

Note that due to the nature of JBOD and RAID 1, the performance of the J4 in these two configurations was basically that of a single internal drive.

Data transfer, Thunderbolt vs. internal (in MBps)
(Longer bars indicate better performance)
Unit to unit  
Self-read and write  
Promise Pegasus R6 (RAID 5)
Promise Pegasus J4 (RAID 0)
Crucial m4 (internal drive)
LaCie Little Big Disk SSD
WD VelociRaptor 600GB
Elgato Thunderbolt SSD
Seagate Barracuda XT
Promise Pegasus J4 (RAID 1)
Promise Pegasus J4 (JBOD)
Drobo Mini

Thunderbolt vs. external
In this test the J4 was used with the test computer, which is how it's most likely to be used in the real world.

In RAID 0, the device again excelled, at 200MBps for writing and 156MBps for reading. In RAID 1 the score was reduced to 111MBps and 110MBps for writing and reading, respectively. Finally, in JBOD, it scored similarly at 111MBps and 95MBps for writing and reading.

Overall, the J4 performed very well for a Thunderbolt device of its type, and remained cool even during heavy loads.

Data transfer, Thunderbolt vs. external (in MBps)
(Longer bars indicate better performance)
Promise Pegasus J4 (RAID 0)
Promise Pegasus J4 (JBOD)
Promise Pegasus J4 (RAID 1)
Drobo Mini (via USB 3.0)

The four-bay Pegasus J4 is very much a combination of two dual-bay storage devices housed in one compact package. If you can overlook the current lack of RAID 5, you won't be disappointed by its performance and ease of use.


Promise Pegasus J4

Score Breakdown

Setup 6Features 7Performance 8Support 8