Thanks for the replies, guys.
As far as restarting, I'm on a large network, and restarting takes two or three minutes. It's the solution I've been using, and it's very disruptive. From my point of view, restarting the computer to swap a CompactFlash card makes about as much sense as disconnecting and reconnecting a TV cable to change channels.
I've been playing with this since posting, though, and I now realize disconnecting the device - the card reader - is overkill. All I want to do is remove and replace the card. To do that I NEED to keep the device connected; once disconnected, it doesn't re-connect by simply inserting another card. I've found I can go to Explorer and EJECT the card (right-click); I get warnings, but if I continue, the system allows me to remove the card and put in a new one. I'm still experimenting with this, so don't take it as gospel, but I think I'm onto something.
Unfortunately, as a photographer I work with image files 99 percent of the time, so I'd be lost without thumbnail or slide show views of the CF content. But the info about the thumbs.db file being the culprit is very valuable. Perhaps I can figure out how/where to find and delete it when I need the OS to release the device. Even better will be if ejecting the card makes removing the device, and chasing down the thumb.db file to do so, unnecessary.
Let me know if you're interested and I'll keep you "posted."