I stumbled into this because my old Sony reader won't used the 5 volt adaptor from my HTC while both usb.:
http://askleo.com/why-dont-all-usb-c...l-usb-devices/
I have a wall adapter, 220 volts comes out for normal use so you don't lost your outlet.
Under it are two usb connectors.
The together power is more then 2000 milli amps.
It is not important because the adaptor only gives what the device needed.
But some devices like my old sony and my Aura HD detects when the adaptor can give more and then ask: is this a computer or do you only want to use the charging.
My old Sony just don't want to charge when there is more then 500 milli amps present.
But also a second thing can occurs... and I have to write this from memory so it can be possible that I explain it the other way around but that doesn't is important for the story.
An usb outlet has 4 lines, 1,2,3, and 4.
Point 1 gives 5 volts, 2 is minus data , 3 is plus data, 4 is zero for loading.
Some adapter makers just put point 2 and 4 together, they are both minus eh...
But then some of my devices detects the cirquit of the charger because there is something happening on the minus side of the dataline and thinks that it is a computer,and comes up with the question.
But there is not coming anything on line 3 so my old Sony "thinks" this is a computer, nothing is coming on line 3 so I just disconnect.
And will not charge.... While the Aura does.
But some device makers do separate in the device point 2 and 4 for safety.
If you put an adaptor on it that has not properly seperated point 2 and 4, some devices will not accept it.
In theory power from a defective charger can come on the + or - dataline it will kill your device, so if the device have a protect cirquit on it and the charger isn't properly wired inside, the device will not accept it at all.
If you have a combination of such a charger, and such a protected device you can get all kinds of problems with device one,that ask you if you want to connect or not with the computer that isn't there at all, or it will not charge,when another device that is not "protected" will accept it without problems.
Some cable makers make only charging cables,most for devices that uses the usb only for charging and have another connector for data, like one of my tablets, it has a I don't know how much wires for data connection for connecting HDMI, docking station, but a micro usb only for charging.
The provided charging cable has only 2 wires in it, so it works as a charger cable, but not for data, but it looks like a "normal one".
When I use that cable on my Kobo it asks nothing,just charges but no data transfer.
Tooks me a while to find that out, because I switched the 2 usb cables and don't understand why calibre doesn't want to send anything to my Kobo anymore...
So there can be a total blending/mixing of all situations when u use some adapters or mixed cables or devices...