I was able to solve this problem with help from PGMusic Support. It turns out that the disc where I had my real drums copied was formatted as "case sensitive". This presents problems when BIAB is looking for files to load. I needed to re-format the drive so that it was no longer case sensitive. Another option would have been to re-partition this disc and reformatting just the part I needed to use.

Initially, I thought I was missing the actual real drums files; however, the way we checked this was to run BIAB from the audiophile disc. To do this onnect the audiophile drive and then point to the real drums on the audiophile disc in the real drums settings. The error did not occur with this set up, letting us know that the real drum files were there.