WM5torage is a free application for PDAs and smartphone with Windows Mobile / MS Smartphone 5.0 and 6.0. Using this application, your PDA acts as a USB mass storage device and you computer will detect it as an external drive (like USB sticks, card readers, MP3 player and so on).
You just have to connect your device to a computer (via USB connection), start WM5torage and activate it. Then your computer should detect a USB mass storage device and will mount it as an own drive. While WM5torage is connected to the computer, your memory card / flash memory is unavailible on the PDA because it needs to be unmounted.
There are different additional options, for example whether your computer may write or only read on it. Also, LED and vibration can show read / write activity. Multiple cards are supported, but only one can be choosen at a time. You can even mount your Flash memory and data memory, but this can be problematic: When these memories are mounted at PC (= unmounted on PDA), your PDA may crash.
The application worked fine on my MDA III running Windows Mobile 6. I could mount my data memory and memory card; mounting my flash memory crashed the device. While mounting my memory card worked fine (4,1 MBs are transfered in ~ 8 seconds), mounting the data memory was very slow and almost crashed my computer.