USB 3.0 Hub RAID Study

Analysis of a Portable Storage Solution

(Last updated: 12/03/2013)

This article discusses the portable storage solution using bus powered hard drives. For the large capacity, 12TB, backup solution see 12TB RAID 0 Under $400.

The purpose of this study is to perform a hands on analysis of a large capacity, high speed, inexpensive, portable, storage solution using a Redundant Array of Inexpensive Disks (RAID).  The motivation for this study is the need for a portable, external, RAID implementation suitable for a photographer on location.  We assume the necessity of having at least 2 copies of all work, even when on location.  With SSD’s becoming more widespread in portable computers we find ourselves blessed with a much faster portable work environment, but more expensive and, for the first time, less storage capacity.  With small, high speed SSD’s taking over internally there is a need for a large capacity, inexpensive, high speed, portable solution for external storage.  Commercial products targeting this niche include the Promise Pegasus j4 and the Drobo mini.  (Disclosure:  Clicking any link on this page to get to, and buying something, helps fund this site. Thanks!)

This study will consider the “USB 3.0 Hub RAID” and a “Bunch of Memory Cards” against the commercial offerings.

To perform this analysis the following equipment was used:

  • 15 inch MacBook Pro with Retina Display, although any of these will work, just make sure your computer has USB 3.0. Another consideration is to ensure there is enough excess internal storage to hold all of the images and video you will create while you are away from your backup drives.  This, in conjunction with external storage will allow for two copies of your work.
  • 4ea – USB3.0 drives (Hitachi, 2.5in, bus powered, 1TB, 7200rpm) although any of these should work, just make sure it’s USB 3.0:
  • 1 or 2 – USB 3.0, AC powered hubs, at least 4 ports, any of these should work, just make sure it’s USB 3.0 and AC powered (the USB port on your computer cannot provide enough power for multiple bus powered drives simultaneously.)
  • Memory Cards:  As many as needed to hold all of the images and video you will create while you are away from your backup drives.  This, in conjunction with your computers internal storage will allow for two copies of your work.  Here is a great source for Memory Cards.  (Disclosure:  clicking any link on this page to get to, and buying something, helps fund this site. Thanks!)


  1.  The array and memory cards were tested for read and write speed with Blackmagicdesign Disk Speed Test set to stress the system at 5 GByte reads and writes.  20 read and 20 write tests were conducted for each configuration.
  2. The arrays were tested for robustness by unplugging the USB port from the computer while active, then reinserting it, and unplugging a single drive while active, then reinserting it.
  3. The number of disks in the RAID 0 striped array was increased until saturation (no further increase in read or write speed could be obtained).
  4. The disks were then tested in RAID 10 for speed and robustness.

All solutions were evaluated against the following criteria:

  • Portability:  Fits in a laptop bag, or briefcase, along with the laptop.  Can you use it sitting in an airplane seat, assuming AC power is available?
  • Capacity: Over 2TB.  Preferably over 4TB.
  • Speed: The internal SSD on the rMBP averages 427MBytes/sec reads and 447MBytes/sec writes while a 4TB G-drive averages 131MBytes/sec reads and 137MBytes/sec writes. At this time we will define high speed as at least 200MB/sec, preferably over 400MB/sec.
  • Cost:  SSD’s cost roughly $1,000/TB while memory cards can be $4,000/TB.  At this time we will define inexpensive as less than $200/TB, and preferably less than $100/TB.
  • Robustness:  We use the term robustness to describe a systems ability to perform acceptably in a wide range of circumstances.
  • Durability: We use the term Durability as the measure of how long a product will perform without failure under a certain set of expected-use conditions.  We expect new/improved storage/bus technology and/or the reduced cost of existing SSD’s to make the solution we pick today as obsolete as a CD backup, or USB 1.1, within about 3 years.

USB 3.0 Hub RAID (Do It Yourself Disk Array) Review:

  • Portability:  With the maximum height of any piece of equipment in the array being substantially less than 1 inch the USB Hub RAID proves to be more portable than either of the commercial solutions. However, with the number of devices involved it does not lend itself well to using on the go, like in an airplane seat.
  • Capacity:  500GB drives will yield a 2TB RAID 0, 1TB drives will yield a 4 TB RAID 0 and the new 2TB drives will yield an 8TB RAID 0 twice the maximum capacity of the commercial solutions.  The fastest 2TB bus powered drives are currently running at 5400rpm, but the speed tests demonstrate that four 7200rpm drives easily saturate a USB3.0 hub plugged into a single USB port.
  • Speed:

RAID Speed Results

    • Single Drive (1 Disk):  The chart was generated by first testing one drive attached to the USB port on the computer.
    • RAID 0 Striped:  We next attached two drives to an AC powered USB hub and created a RAID 0 striped array using the Mac “Disk Utility” program which comes with OS X (2 Disks in RAID 0).  We then repeated the test, striping three, then four drives into a RAID 0 array.  The chart shows a drop off in acceleration between a 2 drive array and a 3 drive array as well as a reduction in speed between a three drive RAID 0 and a four drive RAID 0.  This demonstrates saturation of the USB 3.0 hub attached to a single USB 3.0 port on the computer.  This is the point where you need to be informed that the read speed suffers as the disk fills, but that is typical of software RAID’s, if not all RAID’s, and it’s still fast for the money.  In the test environment pictured, when the disk was half full, we lost 14% of our throughput reading and 2% writing to the disk array.
    • RAID 0 Striped Distributed:  After topping off the Disk Speed Test on one hub we ejected the striped set distributed the drives over 2 hubs, with each hub plugged into a separate USB 3.0 port on the computer.  The results more closely match the 4 times a single disk speed we would expect without hub interference.  We were recording reads in excess of 500 Mbytes/sec when the disk is empty. The layout looks like this:
      Distributed USB 3.0 Hub RAID 100
      In case you cannot see the image on the screen, it looks like this:
      DiskSpeedTestLooks like someone accidentally tested the internal SSD, but no, the internal SSD is not this fast.  All from 4 cheap drives and 2 inexpensive hubs.  In the test environment pictured, when the disk was half full, we lost 29% of our throughput reading and 3.9% writing to the disk array.
    • RAID 10:  Next the four drives were repartitioned into two RAID 1 mirrors that were then striped together into one RAID 10 array yielding 2TB total capacity and a substantially reduced speed, but with the added security of being able to remove a drive from each mirrored pair without interrupting any ongoing processes.  In testing, the array worked flawlessly when we removed a drive from the array.  When we reattached the drive, the system immediately started to rebuild the drive, unfortunately, it takes hours.  Please read Robustness->RAID 10 below.
  • Expense:  For some reason fully encased external drives with a USB3.0 interface and a cable cost less than raw drives with only a bare SATA connector and no cable.  When you add to that the fact that you will have to spend at least a few hundred dollars to encase those raw drives in a RAID enclosure the expense of a USB 3.0 Hub RAID Array seems pretty small.
  • Robustness:
    • RAID 0:  We detached the hub from the computer during testing and received the “Disk Not Ejected Properly” alert, as you would expect.  Once reattached the system recovered normally.  Next, we detached a single drive of the array from the hub while testing was ongoing with the same results.  Once reattached the system recovered normally.  There was no loss of pre-existing data and no need to run “Disk Utility”.
    • RAID 10:  We detached the hub from the computer port during testing and received the “Disk Not Ejected Properly” alert, as you would expect.  Once reattached the system recovered normally.  Next, we detached a single drive of the array from the hub while testing was ongoing.  This time there was no alert. The test continued without interruption.  Once the drive was reattached all operations with the array continued apace, but a background process was initiated that started rebuilding the drive as seen in the screen shot below.  Considering the number of exposed connections involved we would recommend against RAID 10 for the “USB 3.0 Hub RAID” and instead invest in a strong backup policy.  You will find that a disk will start rebuilding with an inadvertent connection failure, easy to happen in a portable situation.  In RAID 0 an inadvertent connection failure causes a “Disk Not Ejected Properly” error.  No mirror is corrupted. No rebuild starts.  Reseat the plug. Continue to work.

DiskUtility ScreenShot

Stack of Memory Cards Review:

Having a stack of CF and SD cards available can satisfy the primary requirement of having 2 copies of all work.  At the end of the day, copy the days work onto the internal hard drive of your laptop and store the CF cards until you return to your backup drives.  This solution ensures 2 copies of your work, but can be more expensive than other solutions, does not allow you to have your previous work available on location and does not allow for processing previous work on the external drive.   Here is a great source for Memory Cards.  (Disclosure:  clicking any link on this page to get to, and buying something, helps fund this site. Thanks!)

Comparison of Portable Storage Solutions:

Promise Pegasus J4 with 4 – 1TB drives Drobo mini with 4 – 1TB drives USB3.0 Hub RAID A stack of Memory Cards
Portablility – 1 Device
– 1 Power cord
– 1 Thunderbolt cord
– 1 Device
– 1 Power cord
– 1 Thunderbolt or USB cord
– 5 Devices: 4 drives + hub
– 1 Power cord
– 5 USB cords
Excellent:  CF and SD cards are small and easily transportable.
Capacity 4TB 4TB 4TB – 8TB possible with the new 5400rpm bus powered 2TB drives. Only possible as temporary backup on location. Purchase only what is needed.
Speed – 484MBytes/sec avg Write
– 500MBytes/sec avg Read
Measured by CNET
– 106MBytes/sec Write Thunderbolt
– 95MBytes/sec Read Thunderbolt
– 59MBytes/sec Write USB3.0
– 77MBytes/sec Read USB3.0
– 264MBytes/sec avg Write
– 306MBytes/sec avg Read
– 420MB/sec avg Write 2 hubs
– 490MB/sec avg Read 2 hubs
– 121MBytes/sec avg Read
– 91MBytes/sec avg Write
But your not going to use it for post processing anyway.
Expense $750:  $369 for the empty enclosure plus $29 for the Thunderbolt cable $700:  $330 for the empty enclosure, includes Thunderbolt and USB cable.  mSata drive optional. $400:  $500 if you opt for 4 – 2TB bus powered drives. At $4,000/TB this solution is only good for temporary backup on location.
Robustness – Thunderbolt only
– Mac only
– All drives must be the same
– Airplane? w/AC power
– Software RAID
– Thunderbolt or USB
-Mix drives, uses total capacity.
– Airplane? w/AC power
– Built-in redundancy
– USB only
– Mix drives, uses smallest capacity
– Airplane? No
– USB only
– 100% bus powered
– Airplane? Yes
– Temp backup only
Durability Presumed excellent since all drives are screwed into place in an aluminum shell. Presumed good since all drives are encased in a single device. Excellent. This author owns drives that have been carried around in briefcases and tossed in drawers for several years. Excellent.

If you rarely travel, and when you do its for only a short while, perhaps a stack of Memory Cards will suffice.  At least you have a temporary backup of your work.  If you have Thunderbolt on all of your computers and don’t want to look like a super geek, buy the Promise Pegasus j4, otherwise implement the “USB3.0 Hub RAID”. Geeks are considered cool these days. A complete description of how to implement the “USB3.0 Hub RAID” on Mac OS X will be forthcoming in the next post.

Although the “USB3.0 Hub RAID” may not be what you want for a portable solution (too many wires) be sure to check the discussion on backup drives. How about a 12TB high speed solution for under $400, or a 16TB for under $600? And since its a backup device, who cares about all those power cords?


4 thoughts on “USB 3.0 Hub RAID Study

  1. Pingback: Promise Pegasus J4 Review |

  2. Pingback: RAID 10 Using Mac Disk Utility |

  3. Pingback: 12TB RAID 0 Under $400 |

  4. Pingback: Creating a Striped RAID Set Using Mac OS X Disk Utility |

Leave a Reply

Your email address will not be published. Required fields are marked *