How to identify CDROM that files reside on - Patching program.

I found out that a particular print run of a program has some corrupted files. As such, we need to either send out brand new CDROMs, provide a 200+MB file that is the equivalent of the CDROM via internet (helloooo bandwidth charges), or make a patch that will install the 10 megs of actual "corrupted" files and copy the rest from the CDROM to the hard drive. I am trying the latter.

My problem is that this is for non-technical users. I want to make a standard patch that will search for the correct drive letter that holds the original CDROM. It will verify that the files are correct by comparing for file "X" in the drive location. I can't assume what CDROM letter it will be, although I suppose that I can include a prompt to say what drive letter that the CDROM is at. I just prefer to make the whole operation seamless and user-non-interactive as possible (idiot-proof, if you will). I have looked around, and can't seem to find the exact idea on the boards. My idea of the code (in non-code language) is as follows:

Set Drive ?:\ to D:\

Begin:

Look in Drive ?:\

If File Verifyfile.xxx exists in directory  Data\, then

  copy files xxx-yyy to directory $INSTDIR

  If Drive ?:\ != Z:\ then

     Drive ?:\ = Drive ?+1:\

     goto Begin:

  Else messagebox "CDROM not found, please insert CDROM and click OK." OK, Cancel #message box entries

    If OK, set Drive ?:\ = D:\

       goto Begin:

    Else Quit:

Thank you very much. I have a general idea of how the script is supposed to be done, but I really don't know how to SAY what I need to the computer. This is helping me get more familiar with how NSIS interacts with the computer.

Thanks again.