My notes on using the Sony NW-S785 walkman with linux, should apply to all NW-S78* models like NW-S784. Looks like this model is sold in Japan only.

This has to be done to do file operations. Do not disconnect the walkman while it is mounted!

tail -f /var/log/messages &    # start watching the logs
# now onnect the walkman, it can be accessed with the usual linux usb-storage driver.
# if everyting goes well usb_storage gets loaded and you the A826 gets available as a new device.
# You should get an output like 'sda: sda1', saying you can access the A826 under /dev/sda1.
# If that doesnt happen try to manually load the driver:
modprobe usb_storage
# - if that still doesnt work check if usb-drivers are loaded and the A826 comes up as usb-device at all.
mkdir -p /mnt/walkman                                  # creates a mountpoint if not existant

# now lets mount the S785, use your device here.
# The option ensures that we properly see UTF8 encoded filenames
mount -t vfat -o iocharset=utf8 /dev/sdc1 /mnt/walkman