I have been struggling with USB card readers since I switched to using RAW files. When trying to transfer large amounts (~4 to 8 GB) of image files from an SDCard in a reader, to my computer, it would get part of the way, and would give an error and disconnect the card/reader. I would have to remove it and plug it back in, then transfer the remaining files. I tried 2 different card readers, one purchased very recently. All of them USB2 (because that's all my computer has) and both cheap brands.
I can transfer the same files without error, by putting the card back in my camera, and plugging it into the USB port, but that's a bit slower. So, today I bought a newer cardreader. Not a big name brand - it's a Digitech, but it's a USB3 reader, so newer design, supports SDXC to 2TB etc etc., and still only cost $15.
So, looks like the older generation cheap readers don't fare too well when pushed hard. Thought I'd mention this one, in case someone is having trouble, or is just looking for a card reader at a good price. (am I allowed to post a link to the where I got it?)