You could also try the following:
# mount /dev/sd0a /mnt
Directory location will depend on your system, so you can change that accordingly.
My third suggestion is to look at a possible usb-mount package for NetBSD. I believe Debian has such a package when you want to mount USB. So you will need the USB Storage module and compile it. After this is compiled, the system should be able to read the flash disk file system.