When you consider that ejecting removable media from the Windows systray has been a thing since Windows 95, you would figure that Windows would offer a better way of telling us why it can't eject a USB/SD/Image device today. There have been times that the dialog window will tell me what program is causing the issue, but more often it gives no clue. One not-so-convenient trick is to use Sysinternals Process Explorer to search for the removable drive letter in use to see the culprit. What do you do if the culprit is explore.exe? It would've been quicker to reboot the computer than to go through the time of searching for the process.
If you keep a few Remote Desktop sessions open, it may be worth it to disconnect them all before attempting to eject media. Turns out that the default behavior of RD is to connect to these devices and they don't release then when the host OS tried to eject. Again, how long has Microsoft been at this! We all know that RD does this, but it isn't always apparent that's the reason it won't eject when you need to.
Since I've been burned by this so many times, it's the first thing I do when my eject is blocked. I'm not at the point yet that I'll remember to do this before I eject but I no longer hit dead-ends when I get the message.
