Typically multi-bay external storage devices tend to utilize 3.5" drives due to the lower cost and higher capacities. The downside, however, is that 3.5" drives are physically larger and heavier, which makes a multi-bay enclosure rather difficult to move around on a regular basis. To fix this, Promise is offering a 4-bay 2.5" RAID solution called the M4.

Promise Pegasus2 Lineup
  M4 R4 R6 R8
Form Factor 4 x 2.5" 4 x 3.5" 6 x 3.5" 8 x 3.5"
Supported RAID Levels 0, 1, 5, 6, 10 0, 1, 5, 6, 10 0, 1, 5, 6, 10, 50 0, 1, 5, 6, 10, 50, 60
Connectivity 2x Thunderbolt 2 (20Gbps each)
Available Capacities 4TB 8TB 12TB & 18TB 24TB & 32TB
DImensions (HxWxL) 4.2" x 5.0" x 6.6" 7.5" x 9.6" x 7.3" 9.8" x 9.6" x 7.3" 12.2" x 9.6" x 7.3"
Weight 5.5lb / 2.9kg 15lb / 6.8kg 20.1lb / 9.1kg 24.2lb / 11kg

Aside from capacity, the M4 offers everything that the R4 does as you get hardware RAID 5 and two Thunderbolt 2 ports for daisy-chaining. The weight comes in at almost one third of the R4's weight and the dimensions are considerably smaller too, which makes the M4 a lot more portable than the rest of the Pegasus2 lineup. Sadly Thunderbolt 2's ten watts of power is not capable of powering the M4, so it is not a fully portable solution like regular external hard drives are.

The M4 is available for $999 in the Apple Online Store and the target market for the M4 and the whole Pegasus2 family is video professionals. Promise markets the M4 as a solution that offers portability for over an hour of uncompressed 4K footage. While there are arguably cheaper and larger external 3.5" hard drives around, the M4 provides redundancy via RAID 5, 6 and 10, which is more or less a must for professional video editing because data loss could end up being very expensive.

Our review unit shipped with four 1TB 5,400rpm Toshiba hard drives. These are 9.5mm i.e. two-platter drives, so we are not dealing with super high density here. Promise told us that they are not offering 4x1.5TB or 4x2TB configurations due to price sensitivity as $999 is quite expensive to begin with, although I am not sure if I agree because I could see video professionals paying more for increased capacity. In the end, 4TB is not that much if you deal with 4K video.

Fortunately Promise has made hard drive swaps convenient as pressing the button on the bay will free the lever, which you simply pull to get the drive out. The drives are attached to the bays by four standard hard drive screws, so any 2.5" drive will work. Officially Promise only guarantees compatibility with the Toshiba drive, although the user manual suggests that the drive does not have to be the same make and model.

Getting inside the M4 is fairly easy. There are a few screws that need to be removed until the top comes off and you end up having access to the PCB along with the rest of the components (PSU, fan, etc.). The RAID controller is covered by the heat sink, so I do not have a photo of it, but I was told that the silicon itself is from PMC with custom Promise firmware. A quick look at PMC's RAID controller lineup suggests that the silicon is the PM8011 SRC 8x6G, which is an 8-port SATA/SAS 6Gbps controller with a PCIe 2.0 x8 interface. 

Like many Thunderbolt devices, the M4 has two Thunderbolt 2 ports for daisy-chaining. The Thunderbolt controller is Intel's DSL5520 with two Thunderbolt 2 ports (i.e. four channels) and it connects to the RAID controller through a PCIe 2.0 x4 interface. Intel lists the bulk price as $9.95 on their ARK site and the TDP is 2.8W.

Test Setup

Unfortunately I do not have a Mac with Thunderbolt, so the results and analysis are limited to a Windows based system. Based on what we have talked with manufacturers, there is some difference in performance between Thunderbolt in Windows and OS X. A part of that comes from the fact that in PCs, the Thunderbolt controller is connected to the PCIe lanes from the PCH, whereas in Macs they come directly from the CPU. The Windows drivers are also not as good as the native OS X drivers, which I guess is not a surprise given that Apple has always been the biggest supporter of Thunderbolt. Either way, the results should represent performance under both OSs as long as we are not close to saturating the interface.

CPU Intel Core i7-4770K running at 3.3GHz (Turbo & EIST enabled, C-states disabled)
Motherboard ASUS Z87 Deluxe (BIOS 1707)
Chipset Intel Z87
Chipset Drivers Intel + Intel RST 12.9
Thunderbolt Adapter ASUS ThunderboltEX II/DUAL
Thunderbolt Drivers
Memory Corsair Vengeance DDR3-1866 2x8GB (9-10-9-27 2T)
Graphics Intel HD Graphics 4600
Graphics Drivers
Desktop Resolution 1920 x 1080
OS Windows 7 x64
The Pegasus2 M4: Software
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  • joystone324 - Sunday, September 14, 2014 - link

    no man
  • johnny_boy - Monday, September 15, 2014 - link

    Why on Earth couldn't they make this thing use less than 10W? Each drive uses less than a watt so that leaves 6 watts left for the controller. It could be done entirely through software off an ARM SoC running linux!
  • repoman27 - Monday, September 15, 2014 - link

    This unit ships with 2.5-inch spinning disks that draw 2.5 W each when in use and over 3 W at startup. Even with staggered startup, you're at 10 W right there. Then there's the PMC-Sierra RoC, which judging by the heat sink might draw around 5 W without counting the SDRAM and NAND, and the DSL5520 Thunderbolt 2 controller which needs 2.8 W. There's also a rather large PLD and a Parade PS181 DP to DP++ converter that need juice. Since this is a two-port design, the PSU also has to be able to provide 10 W of power to the next device in the chain.

    Speaking of the 10 W power budget, that also has to power an active cable, which only leaves "up to 8.5 W" for the device. Since the original Thunderbolt cables could draw over 2.4 W all by themselves, and the lowest power Thunderbolt 2 controller uses another 2.1 W, you can really only count on roughly 5 W for your device design unless you use a tethered cable.
  • philipma1957 - Monday, September 15, 2014 - link

    I own an r-6 and it has been a beast. One of the most important features of the r-6 is the drives are bootable. I own the smaller 2.5 inch 4 drive model the j4 and it is not bootable. the j4 fan was loud and whiny it sets in a box doing nothing. It was designed to stack under a mac mini and it was very disappointing purchase for me. I would hate to buy this unit and find it is not bootable. and has the same whiny loud fan. And if I was doing video since I have to plug this into a power source I would just get an r-4 or r-6 and use that. The r-6 can have a lot of setups that are fast and back up your info.
  • mschira - Tuesday, September 16, 2014 - link

    this is silly. Why would I bother with a RAID setup for 4 TB, when I can get a 4TB single platter in 3.5"?
    That's gonna be smaller than 4 2.5" drives, consume less power, and on goes the list of advantages.
    Yea, it's somewhat less fail tolerant because I can't use RAID 5, but it will fail much less often because it is only ONE drive rather than 4 drives.
    But seriously? RAID 5 will give you 3TB. How about using two 2TB 2.5" drives in RAID 1? Will be more compact, fail less often.

    Now use 2TB 2.5" drives (i.e. 12mm ones) and the situation starts to turn. You can build a 6GB RAID 5 setup with good speed fault tolerance and a reasonable size.
    May not be a magic bullet in many occasions, but at least in a few situations.
  • RedHunter2386 - Thursday, September 18, 2014 - link

    Please keep in mind that PMC is part of Promise Technology and is also known at the "OEM"-brand of Promise. Therefor it is not that strange they would use their own chip in their devices, even if the specs are higher than the unit actually demands.

    On the Promise booth at IBC (in Amsterdam), they were showing the Pegasus M4 with 4x 512GB Samsung 840 EVO SSDs. This was a demo unit, as there is no official SSD version available (yet). The speeds in Blackmagic Speed Test were showing around 1100MB/s Write and 1150 MB/s read. That's quite a difference from the speeds mentioned in this review.
  • stux - Tuesday, September 23, 2014 - link

    I think this review needs an update or a Redux

    For a TB drive enclosure its absolutely imperative to test it on a Mac with TB2, or you might as well not bother, especailly with the reports for 1100/1150MB/s performance with 4x 840 Evo SSDs.

    Drive noise is a very big concern with a product like this. I've seen dB meter apps for iPhones, and audio engineer friend of mine raves about one and how its with in point something something of his lab meter. Cheap, and cheerful and would help a lot with objectively quantifying the noise.

    This drive sounds like its trying to compete with the Drobo Mini, which has 4 2.5" bays and an m-sata 5 bay which can act like a cache card, and is USB3 and TB

    I'd really really like to see a performance comparison between the drobo mini and the the M4 with 4x 1 or 2TB HDs and also with SSDs. That would be very interesting, and very valuable to mac professionals trying to work out high performance or high capacity storage solutions for portable scenarios.

    Thanks Kristian, I did enjoy the review :)
  • jonb8305 - Wednesday, October 15, 2014 - link

    Someone should do an update with the official SSD version from Promise http://www.promise.com/promotion_page/promotion_pa...
  • PeterBr - Sunday, May 3, 2015 - link

    I just placed an order for a PROMISE Pegasus2 M4 4TB at $1500 (this is a 2.5" disk version), and now Im thinking I could have purchased a 3.5" version from another company with more TB installed for the same price. Could anyone recommend a better prices/deal and reliable 8TB or higher raid drive with thunderbolt for 4k video editing?

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