So far, no luck with any suggestions. I had several IT people with Macs working on it, but they could not access or delete the partition on the flash drive.
Note my previous error message from DISKPART, which reported a discrepancy between "Current Read-only State : Yes" and "Read-only : No."
On the web, I found another person had asked this question, but it was diverted to Microsoft support, and the answer was not published.http://www.eggheadcafe.com/software/aspnet/35994039/disk-marked-as-current-readonly-but-readonly-flag-is-off-.aspx
I've tried to write-protect/then unprotect the drive, and put it in the freezer to see if I could get access to reformat, but failed. I also exposed the flash drive to a strong magnet, but the files remained intact. Apparently data is not stored magnetically, so degaussing would not work either.
To delete my proprietary data, I may have to boil it or microwave it, unless someone has another idea!