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I am looking for a HDD enclosure which can make two 2.5" drives available from a single USB port, while not using any external power adapter, i.e. powered entirely from USB port.

I failed to find any enclosures on the market which are both dual 2.5" and have no external power. Are there any practical obstacles for such hardware to exist?

I am aware that USB 2.0 delivers 500mA per specs, while USB 3.0 should deliver 900 mA. Two HDDs I have at hand require each 310mA and 380mA correspondingly--so USB 3.0 should have enough power to run them them both simultaneously. In theory, even 2.0 should allow to run one at a time--although it's hardly a popular demand, so I think USB 2.0 dual enclosures without external power are very unlikely.

So why no USB 3.0 dual 2.5" HDD enclosures requiring no external power are available on the market?

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Any comments on why my question was downvoted, please? (it's my first question at StackExchange sites, so I'd like to learn if I did anything wrong) –  yurkennis Sep 1 '13 at 10:16
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2 Answers

up vote 2 down vote accepted

Technically it should be possible when both drives never use more power than the USB port can supply.

However most 2½ inch drives seem to draw around 500 to 600 mA, which is more than a single USB 2 port can supply. Using only a single, extra low power drive or two USB ports *1 to supply power is the normal solution. Anything else required an external PSU.

This made it unattractive to build a dual 2½ disk enclosure. Those would require an external PSU, competing with external 3½ inch disk enclosures. And the larger disk enclosure used to be faster and larger.*2

Still, there is no technical reason why one could not build a dual 2½ inch disk enclosure. Especially not with the 900mA limit from USB3. It is just that it would serve a very small niche market. And that makes it commercially unattractive.

*1 IIRC using dual USB ports to supply more than 500mA power is not even officially allowed by the USB standard.

*2 Also, it is possible to use a 2½ SATA disk in a 3½ inch disk enclosure. Thus further reducing the market for it.

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It is mA, not mW... 500 mA at 5v = 2500 mW. –  psusi Sep 1 '13 at 3:06
Thank you. Corrected. –  Hennes Sep 1 '13 at 13:55
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I had the same question and finally sourced the Cineraid Model CR-H212 which I bought from a supplier on EBAY (shipped from the US to UK). I then bought 1TB SATA II hard disks from Amazon. It was very simple to install the disks in the enclosure.

I now have one unit which I have set up as RAID-1 that I use at work - this is for vital data so I need an instant backup. I have another unit at home which I have setup as a 2TB storage.

And yes, the dual 2.5 disks are powered only by the USB port :)

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Thanks for sharing. Does it work with USB 2.0? How does it resolve the case when both disks are accessed simultaneously (and therefore require more power than USB 2.0 can give)? –  yurkennis Oct 31 '13 at 14:05
Hi. Yes, my instruction booklet states external host USB 3.0 - compliant to USB 2.0 and USB 1.1. You are given a main USB 3.0 cable which connects to a Micro USB slot on the enclosure. Also you are give a secondary USB cable with which connects to a 5v power connector on the enclosure in the event that you need extra power - personally I have never had to use it, and I mainly use USB 2.0. –  Martin Oct 31 '13 at 17:05
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