Note: While my old SanDisk Cruzer Glide USB flash drive is listed as type “Local Disk” in Windows 10 Explorer, nevertheless Rufus was able to create a bootable drive out of it from a Parted Magic iso file. Macrium Reflect also created a bootable rescue disk from it, and I expect Windows 10 itself would create one as well. So my problem was that UNetBootin simply did not recognize a USB drive if it appeared as type “Local Disk”. I submitted this as an issue (#155) to the UNetBootin GitHub repository.