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?