Use DSynchronise, as it’s free! The reason why it’s a good option for you, is that it doesn’t do what Windows Explorer does by counting the amount and size every file in the queue before copying. It just copies straight away.
However you can tick the checkbox so that it will count the disk space first. You can also choose to store a backup of every file that is deleted or overwritten in advance. And you can use preview mode so you can test how the synchronise will occur before you actually do it for real.
Also keep in mind that it doesn’t always copy files in alphanumerical order, so if the copying or synchronising suddenly stops halfway to your detriment, then you might have to start again from the beginning.
I find that the old version 2.30.1 is easier to use and faster than the newer version (that was 2.41.1 at the time).