Was not sure where to put this as I believe it may occur for Windows in general, but since I am on Windows 10 Pro, I landed here.
I found a quite unexpected process preventing the Safely Remove USB mechanism from working. The process is named CMigrationService which appears to have been bundled with Samsung Magician SSD software beginning with Samsung Magician 7.2.0.930 (AFAIK). I found the CMigrationService holder using Windows Event Viewer (Windows Logs > System > Event ID 225). The CMigrationService is executing MigrationService.exe from within the Samsung Magician 7.2.0.930 software.
Unable to stop this process via Windows Task Manager but able to disable the service AUTO start using CCleaner.
I wasn’t using Samsung Magician at the time and had not enabled it to run at Windows startup. I had no idea Samsung had begun bundling a Data Migration component within Samsung Magician as they had always been separate.
I am considering downgrading to an earlier version of Samsung Magician as I do not want to deal with this Data Migration feature (bug?).
If anyone is having problems with USB Safely Remove and have Samsung Magician installed, you may wish to investigate if any relationship.
I ran into this in Windows 10 Pro (64 bit) with Samsung Magician 7.2.0.930 installed but not started or in use (or so I thought). The USB device was an external WD HDD.
Almost forgot. Just for good measure, this Data Migration service has been accumulating small log files daily in its LOGS folder – apparently indefinitely with no cleanup.
FWIW as a sharing topic. Unsure if anyone will find this useful (although using Event Viewer to find out what held the USB device was useful to me).