If its Virtual Box, are you sure you've installed the guest essentials? This is a requirement for Virtual Box to pick up external folders/shares from your home OS. I'm fairly sure this includes USB devices:
These provide an easy way to exchange files between the host and the guest. Much like ordinary Windows network shares, you can tell VirtualBox to treat a certain host directory as a shared folder, and VirtualBox will make it available to the guest operating system as a network share, irrespective of whether guest actually has a network. For details, please refer to Section 4.3, “Shared folders”.
In the "Devices" menu in the virtual machine's menu bar, VirtualBox has a handy menu item named "Insert Guest Additions CD image", which mounts the Guest Additions ISO file inside your virtual machine. A Windows guest should then automatically start the Guest Additions installer, which installs the Guest Additions into your Windows guest. Other guest operating systems (or if automatic start of software on CD is disabled) need manual start of the installer.
Although, this doesn't appear to be a thing for Mac right now:
Mac OS X guests:
VirtualBox does not provide Guest Additions for Mac OS X at this time.
This forum might help: https://forums.virtualbox.org/viewtopic.php?f=8&t=45349
The way that I have found to solve this problem that seems to work almost every time is:
- Insert the USB drive,
- wait for OSX to mount the drive,
- eject the drive in Finder, and
- then select the Devices|USB Devices menu and select the USB drive to mount the drive.