OK, you got us There is no vfat or msdos support in the ASL Kernel, because this is considered a security flaw. Use scp to transfer files from/to your ASL.
Don't forget to enable ssh before trying this, and for copying files from the ASL you got to make a packetfilter rule to allow packets with source address=outgoing-ASL-interface and dport=ssh to leave the ASL.