The most common cause of all seems to be Adobe's #*%^ing software interfering with Windows, althought it is clear that other software makes the same mistake.
Resources:
http://www.eggheadcafe.com/software/aspnet/31563340/usb-flash-drive-not-appea.aspx
Describes a tortuous troubleshooting process that fingered Adobe Photoshop Album Starter Edition as the culprit. Indeed, when the problem occured today that is the program that pops up when I plug in my thumb drive.
http://www.techspot.com/vb/all/windows/t-6762-USB-Flash-Disk-not-appearing-in-Explorer.html
Describes the many alternate solutions, such as assigning a specific letter to the drive, rebooting, using a third-party version of explorer, etc, and the IE6 workaround.
***WORKAROUND*** is to use IE6 and type "My Computer" in the address bar. The reason why this works is that the fresh copy of IE is forced to check the partition tables (or something like that) and sees the new drive, whereas the Adobe software is blocking previously started copies from doing that, including the copy which is the desktop. THIS DOESN'T WORK FOR IE7 OR MOZILLA.
***Clensing the system**** The culprit process is apdproxy.exe. Deinstall it, or change the service startup to manual. If at all possible, remove as much crap from Adobe as can be removed and still leave PDF reader.
