I'm experiencing the same problem: My ethernet-connected PC running XP can't see the thumbdrive attached to my DIR-655 using Sharepoint (like the other posters have said, it doesn't see the dlink router either) but my wireless-connected laptop running XP can see it just fine using sharepoint. I've tried un-installing and then re-installing Sharepoint on the wired computer, rebooting everything afterward, etc but that didn't help.
If anybody has figured out how to fix this would love to hear suggestions -- thanks!
-m