Hi, a background process may still be accessing the USB even when no files are open.
Please wait for any file transfers to finish, save your work, then try this:
Press Ctrl + Shift + Esc to open Task Manager.
Find Windows Explorer, right-click it >> Restart.
Try ejecting the USB again. Wait for “Safe To Remove Hardware” before unplugging it.
If it still says “device in use” and you need to remove it now, shut down the PC normally and wait until it is completely off before unplugging the USB .
Microsoft’s safe removal guide: https://support.microsoft.com/en-us/windows/hardware/safely-remove-hardware-in-windows