It happens to me every time I unplug the USB drive.
I used the USB drive that came with my UM3, Sandisk 4GB,Sandisk 8GB,a USB memory card reader that had a old 256 MicroSD card,8GB and 16GB memory cards.
The dump Files wont write to the USB drives, It throws up some sort of error,If you power cycle the machine where it can see the USB drives it will write the dump files just fine.