Okay, it was definitely something installed. I finally restored the main S1596-01 to factory settings, re-installed the two USB mass storage driver files, and the USB stick finally showed up.
(I swapped out CF cards with another unit beforehand and made it my main unit, and that one still recognizes the USB stick. Still don't know what blocked it from being detected before, but this result makes me suspect that it was a registry change somewhere.
)
BTW, doing a factory restore requires selecting the Registry Initialize button in MainteTool.exe, which restores the registry back to factory settings. Just disconnecting all batteries doesn't work because the device automatically restores the saved registry settings during reboot. Kinda nice feature for dealing with power loss.
