You might be able to do dmesg and see the output.
As for da0, if you do camcontrol devlist that will verify da0 is your USB stick. It probably is, but no reason not to be 100% sure. Often, if you get that error with the USB stick it's not a good option for FreeNAS. FreeNAS works best on 4GB+ USB sticks(not an SD card in a USB converter or anything else like that) and only name brand products. I've had 2 or 3 USB sticks that weren't name brand that work just fine but because of their excessive latency with random writes it caused errors like yours. I knew better but since it was a test bed for testing for bugs I didn't care until I couldn't use FreeNAS reliably.
