You might have chosen a counter example. If the computer has only one disk controller, traffic on one drive will slow down all drives, depending on how many data paths the controller has.
Your point is good, though. For example, in W7, if the USB disk "ejector" refuses to eject a disk because it finds that some app is using the disk, why can't it tell me which app? (Maybe W8 corrects this issue, in which case, I take it back.)