It's odd when you update your own post... but after pestering Sony Viao support till they elevated my online chat problem, I finally got somebody who seemed to know something. I was having this problem on NTFS USB devices, not on FAT32s; he steered me to Power Management / Advanced Power Plan / devices menu, USB, Power management, where two options: battery, and plugged in, are visible. If either of these is DISABLED, the power management software wants to hold on to the drive, which gets in the way of the safe removal dismount. So I set these to ENABLED (out-of-the-box defaults had been Enabled for battery, DISABLED for plugged in), rebooted... voila.
Hope this helps you folks.