Reader Juliet is a Windows XP user who's frustrated by this problem:
"When I try to disconnect a flash drive or external drive, about 50% of the time I get the message, 'generic volume cannot be stopped at this time.' Usually I just leave it in, and eventually I can disconnect, or if I have to, I turn the computer off."
I assume you mean you receive that error when you attempt to use Windows' Safely Remove Hardware option, which is indeed the right step to take before unplugging any USB drive.
So, why won't Windows let you remove your drive? Snarky answer: Because it's not a very smart operating system. Useful answer: Because some file, program, or system operation is accessing the drive--and removing it could cause problems.
For starters, close any Explorer windows that might be open. Even if they're not showing the drive's contents, they may be "looking" at the drive.
Likewise, close any programs that might be accessing files or folders on the drive. For example, if you have a Word document that resides on the drive and that file is open in Word, close it--along with Word itself.
Where this gets tricky is with programs that might be accessing your drive without your knowledge. For example, maybe your anti-virus utility is scanning or monitoring the drive. That would make "safe removal" nearly impossible. (Aha! Another reason to avoid security software altogether.)
If indeed you have any software that monitors your system, check its settings to see if you can exclude external drives. Granted, such drives can be the source of malware infections, so there's a bit of a Catch-22 here.
Because this problem can indeed be difficult to pinpoint, you have a couple final options. First, you can do what you sometimes do already: Wait until you've shut down your PC and then pull the drive. Second, you can throw caution to the wind, check the drive light(s) to make sure there's no activity, and then yank it.
If anyone knows of a more reliable solution to the problem, by all means share it in the comments. We're all in this together, peeps.