Pi2: External USB card reader ... no recognized

I’ve an external (powered) USB card reader with a SD-card inserted. When the SD card is inserted while booting I do not get any additional USB drive icon on the desktop and I can not access the inserted cards.

When I boot without the cards all four USB drive icons are shown and when I insert a card I can also access the data on those.

That seems strange to me …

Are you getting a rainbow coloured square in the top right of the screen?

No – the coloured square means, that the power consumtion is too high ?

I put the card reader to an external powered USB hub – otherwise the Pi2 does not boot at all and shows strange images.

By the way: the Pi2 also does sometimes NOT boot (does not show the desktop) when the cards are inserted (as of today when I wrote this message).

When the cards are not inserted the systems boots fine, all icons are shown and after inserting them I can work with them without problems.

I think it means that not enough power is available to correctly run the system. Power comes in two forms – voltage and current (amps). To run the Pi, you need both. It might sound obvious, but far too many power packs can give you 5.07V or 2A, but not necessarily both at the same time. ;-)

You will need a power supply capable of a proper 2A. There is not much point in going for something larger as the Pi still uses µUSB which is rated at ~1.8A on the power pins (according to product data for the socket in a catalogue).

That said, you shouldn’t need the full 500mA across the USB pins unless you are using a spinny-rust harddisc or something like that. According to USBinfo, my keyboard and mouse are using 98mA apiece, and a crappy old USB key is also using 98mA. I have a little hub in there claiming to need 100mA, but this would not be necessary on a B+ or Pi2 as it comes with more ports as standard.
So, 98mAx3 is just under 300mA, leaving a potential 1.5A for the rest of the Pi2. The Pi2 appears to consume around 280mA idle and 420mA when using all four cores; and when recording 1080P video (on a single core) it runs at 350mA!
[ http://raspi.tv/2015/raspberry-pi2-power-and-performance-measurement ]

Assuming a good power supply and, most importantly, a good quality cable, of course.

As I said: all stuff is connected to an external powered USB Hub (even my mouse and keyboard), so I do not think, that there is not enough power to the system and no coloured squared image is shown at startup (though I’ve seen this when to many devices are connected to the Pi2).

I have exactly the same problem. If the USB drive is plugged in on start-up, it is not recognised. If I then unplug it and plug it back in again, it is recognised without a problem. I have a 3A power supply and no rainbow square appears.

Jochen

Is this a B+? If so, could this be something induced by the power protection circuitry on the USB ports? This prevents back-powering, but maybe it also closes off data. My B+ will not recognise a wireless keyboard dongle over a power cycle. Just a stab in the dark.

This is a Pi2. I had no problem with my B+ or indeed the Pi1.

Jochen