Wipe free space was selected for all drives including a Maxtor 1TB drive connected via USB.
My computer's internal drives completed without problems, wiping only the free space, but the external drive's format was damaged and the contents were lost. Despite the best efforts of Runtime's GetDataBack NTFS, the recovered files all contained garbled gibberish.
I have accepted the loss of these files, some 16 years worth of accumulated of data and media files, and seek nothing more than working out the exact cause of the data loss, so that it doesn't happen to anyone else.
Please let me know what information would be useful to Piriform for diagnostic purposes.