Just note however that, according to your message, it is unlikely that it is actually explorer.exe itself that is the issue, as the (r) you see means it's only requesting read access, which shouldn't prevent Rufus from getting exclusive access to the volume. Most likely the actual conflict comes from one of the internal Windows processes or services, that are high level enough that they can't be listed (while Rufus tries to help by listing the processes accessing the drive, there's only so much it can do against Windows privileged processes, which are incredibly hard to enumerate). Most likely, you'll want to check that you aren't using one the the applications that is listed at the end of the Rufus FAQ, as some of these are known to be a bit too trigger happy with accessing volumes that they shouldn't be accessing when they have no business to do so.