Wow, that IS odd! My workflow is to do the mix in the DP-24, but just listen with the mastering filters, not record with them. I then connect via USB, and copy off the mix onto the computer, where I do the final edits and mastering with Sony's Sound Forge Pro and iZotope Ozone. That's why I've never used the export function. I did remember reading about import and export in the manual, and I thought I remembered they both use the Audio Depot folder.
Good luck! Hope you find your file!