Your drive is either not online or missing drive letter on Vista 32. Do the following.
Press WinKey + R key and type diskmamt.msc on RUN dialogue box. Disk Management will open.
Look on the right pan of the window for your drive. If shown “Offline”, right click on the drive header and choose Online from the drop down list. If the drive letter is missing, right click on the drive header and choose “Assign Drive Letter & Paths”. Now choose letter W as your new drive letter. Don’t assign letters D, to P or you may get the same problem again.
If this happen on Win7 PC, you can do the same.