No longer getting automatic windows drive letter assignment

Before upgrading to Parallels 9, my Windows XP installation running in Parallels used to automatically assign drives 'Y' and 'Z' for access to OS/X files. I have used these drive letters extensively for statistical analysis software (which is the main reason that I use Parallels).

Now that I am running Parallels 9, those drive letters are no longer being assigned. I can still access my OS/X files from within Windows, but the drive letters aren't being assigned. This has broken my analysis code. I don't know exactly what files the drive letters were linking to so I can't even fix this manually. In any event, I shouldn't be required to manually assign drive letter codes . Re-writing all of my analysis code is an unattractive option.

Did Parallels change the assignment of drive letter with Version 9? If so, why? And what work-around is there to restore the previosu behaviour?

Thank you.