I haven't experienced this but the only though I have is did you try a reboot to see if everything returnS to normal?
I can't see what McAfee would do to prevent external drives from being seen other than offering to scan them as they are plugged in.
Other than that all I can suggest is Technical Support linked Under Useful Links at the top of this page. It's free by phone or online chat.
For now, if I stop and restart Explorer.exe, it will release any drive disconnected yet still appearing in the explorer window with the red question mark. Additionally, if I connect a new drive, it is recognized in device manager and under the manage window in administrative services, however, not in windows explorer window. If I stop and restart explorer.exe, it will become recognized in the explorer window.
You commented that "I can't see what McAfee would do to prevent external drives from being seen other than offering to scan them as they are plugged in." However, afterdoing some searching on the forum, I have observed various problems in the past with explorer.exe related to poorly written code in various forms of Mcafee products, so there certainly is the probability that this is such an issue. Obviously, my particular configuration in some way must have a relationship if others do not experience this issue in the same manner.
I agree that it certainly could be connected with the software but it's just I've never experienced it or dealt with a similar instance that I can recall.
Technical Support may be better assistance.