This device is in use when trying to unmount external hard drive or USB thumb drive

This device is in use when trying to unmount external hard drive or USB thumb drive.

For me this issue was caused by AMD User Experience program. I used the following steps to turn of this feature. Note that this feature is opt-in in the first place. I turned on the feature after the driver installation was finished to help AMD engineers diagnose problems.

Turn off AMD User Experience program:

Right click on your desktop

1. Select AMD Radeon Software

2. Navigate to the Settings icon (gears icon)

3. Click the General tab

4. Scroll all the way down

5. There is an item down the list called AMD User Experience 

6. Click this item and select Leave

Try to eject the drive again, now it should work as intended.

If it still didn't work, or if your PC has no AMD graphics, you can download Microsoft's Process Explorer to investigate further. You can download it here.

To find out which process is stopping your USB attached drive from ejecting, start Process Explorer and then click on Find > Find Handle or DLL...  This opens a search window where you can input a search string. Then you can input the drive letter that the USB attached drive has. For instance E:/. After clicking search, you should see the process that is using the drive.