I can never eject a flash drive on the first try. Why? It may be that it is being indexed as a dot appears in spotlight. However, it doesn't matter how long the flash drive has been plugged in, nor the size of it, it will always fail to eject on the first try. You would think that after a while it would finish indexing it and cause no problem, so that may not be the issue here.

Any thoughts?
It could be that you have open applications using files on your flash disk. 

You could try running the terminal command "lsof /Volumes/_NAME_OF_FLASH_DISK_HERE" and if you see an output, that means you have open/active files on your flash disk.
You need to close those and you can now eject.

Hope this helps,
Ismael Casimpan