ok, so run the mount command first, and find where your USB drive is mounted. the you can run:
cd <mount point of usb>
and that gets you to your usb drive.
but if you want to find big files, then you can run:
find <usb mountpoint> -size +100M
-replace <usb mountpoint> with your actual mount point. it might be /mnt/sda1 or /media/sda1, something like that
this command will find any file larger than 100 MB