Some USB sticks not mountable in Kubuntu inside Virtualbox

Hi,

I cannot mount some USB sticks in my virtual Kubuntu using Virtualbox.
Let me explain: I use MacOS (on a MacBook Pro), installed Virtualbox, and I use Kubuntu 14.04 64 bits as a virtual guest.

I often use USB sticks, mounted physically on my Mac, but readable only in Virtualbox (I even formatted encrypted sticks directly from Kubuntu, and they bypass the Mac to be read directly from Kubuntu, using some options of Virtualbox).

BUT I can never read "large" USB sticks. They are never recognized anyhow.
The largest stick I could use was 4GiB. In contrast, my new 64GiB stick is not recognized. And my external USB hard drive (1GiB?) either.

Note that they do appear in Virtualbox when I right-click the USB icon. But, once I enable the device by clicking on it, nothing ever happens in my Kubuntu (in contrast with smaller drives).
Any idea?

Note that I have not installed USB 2.0 support (once I had installed it in Virtualbox, it crashed my virtual guest as soon as I plug a USB device). Can this be related to that problem?

Thank you.