Unmount Disk is a small utility that help you to unmount and eject your flash and hard drives in one click. Use it every time before physically ejecting your drives and you never will see "Couldn't unmount disk" error.
1. I have never seen this problem either. If you properly eject a disk by dragging it to the trash or highlighting it and using command e, you should not have a problem.
2. There is no free download for trial as stated on the developer's site. You are taken to the app store and asked for $1.99 for a useless piece of software.