Determine what application is using your external/USB drive so it won't eject

aka. determine what app has a file locked (keep a file in use)

Process Explorer (press Ctrl+F then type drive letter of the drive)
source: Which programs are using my USB flash drive?

alternate way:

FFB - Flush File Buffers

From Computer Management \ Disk Management, make the drive offline then re-try to eject
(Next time you plug it, remember to make it online again before use)