Windows can't stop Generic volume

I am using Window Vista on my computer and I have used the backup feature of this same operating system to backup some data on my external hard drive. It went smoothly and everything was copied but when I tried to safely remove the same drive then it was giving me some message - " Windows can't stop your Generic volume device because it is in use. Close any programs or windows that might be using the device, and then try again later". I have checked that everything was closed and ensure in task manager that this hard drive was not using any process. After about an hour or so, I was finally able to unplug it safely? So, i was wondering what could be the problem here?

I think that the backup must be compromised so check to make sure whether there are data backed up or not on the external drive. If you disconnect a drive and if it gives the message that it cannot be stopped then this means that the drive is in an uncertain state. You have to wait for the "safe to disconnect" message or either shut down the whole computer which will close the running process and no data gets damaged as well on the external drive.