It is possible that the drive has become corrupted. It is unlikely that the OneCare scan caused the problem. I suggest trying the following:
1. Disconnect and reconnect the USB drive; check if the files now appear.
2. Reboot the PC; check if the files now appear.
3. Open Windows Explorer, right click the USB drive and select properties, then the Tools tab and the Check Now button in the Error Checking section.
If none of these help and you believe that OneCare caused or contributed to the problem, please contact support.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2