There are a couple of things I can think of. The easiest and most reliable would be to use an .inputbox to have a user type in the drive letter or an application.getopenfilename to select the first file, then get the drive and folder name from that.
If that is not an option, the below code will go through each drive A:\ to Z:\ and search for the first file you list in your code above. Once found, it will make the variable strFound equal to that DRIVE (ex: P:\)
After putting this code first, you'd have to alter the rest of your code to use strFound:
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