How to safely eject external

[admin]

That is indeed strange. Aside from user processes, are there any system processes (such as svchost.exe) accessing the drive? Is this USB drive the source or the target drive?

Maybe you can try taking the drive offline in Windows Disk Management and then ejecting it.

Resource Monitor watching D:\ was quiet.   USB drive is the target.
How do I use Disk Management to take drive offline?

Right-click the disk header, then select "Offline" from the pop-up menu, but this does not apply to USB flash drives.