System Restore error: "Changes made to drive G: after this point cannot be reversed because the drive was either excluded from system restore monitoring or was turned off or removed."

I have been having some issues with my computer recently that I figured could be solved with System Restore.  So I went into System Restore and went to a restore point that was a couple of weeks back.  After I clicked on it and pushed the ok button, the following message popped up: "Changes made to drive G: after this point cannot be reversed because the drive was either excluded from system restore monitoring or was turned off or removed."  I let it continue and do the restore.  My computer rebooted and came up with a System Restore error that said the restore failed.  The same thing happened with other restore points.  I don't know why I'm getting this error, because I haven't made any changes to this drive except for copying a word document over to it.  I thought that System Restore doesn't monitor removable disks.  Does anyone know what is going on here and how I can get System Restore to work without the errors coming up?