problem is more likely to be in the Virtual Box USB driver rather than in the XP virtual machine.
1) Does the USB stick mount on the HOST without problems.
2) USB3 mode not available in XP AFAIK so that could be a problem although USB devices can operate at USB2 or even basic legacy USB mode (older printers etc).
Can't say I've had probs with 16GB USB sticks (NTFS formatted) being attached to XP Virtual machines (although I'm using vmware).
3) Check in the VM menu after XP is powered on if the USB device can be removed from the Host and attached to the VM.
I'm sure VBOX like VMWARE has a similar menu where you can attach / remove devices to the VM.
Here's the VMWARE menus I'm sure VBOX has almost the same.