Problem:I am using VirtualBox 4.2.x on OS X 10.7.x running an Ubuntu Guest. When I try to plug in a USB device, such as a USB drive, USB MIDI keyboard such as the Keystation 88-es, etc., VirtualBox shows the device as captured (with a checkmark in the USB devices list), yet Ubuntu can't find it. I've already tried things such as installing VirtualBox Guest Additions, setting proper user permissions, etc.
If you've managed to mount and capture the USB device but it's still not showing, you've probably already made it past most How-To's Googling will offer. One bug fix that seems harder to come across is this: Set the number of virtual CPUs to 1.
Apparently after VirtualBox version 3.2.0, a bug has caused USB devices to not show up in some Linux-based guests when the number of virtual CPUs is set to more than 1.
If this obscure bug fix doesn't help, check out the references below to lots of other potential solutions to the USB-won't-mount problem.
I realize that this post isn't complete, however it was this one somewhat obscure bug that caused many hours of attempted debugging and Googling until it was found. So to save others the trouble of finding it buried in a page full of instructions and fixes, I opted to highlight only this one bug, and link to other pages full of good solutions.
Hopefully by the time you've reached this blog page Oracle, VirtualBox, Apple, or some others have managed to fix the problem!
This workaround is version specific so your mileage may vary depending on the version of OS X, VirtualBox, VirtualBox Guest Additions, Linux, etc., that you are running.