Always backup before resizing a partition. These procedures usually proceed without issue but occasionally things go wrong with unpredictable results. Of course you should have a backup anyway. There are many ways you can loose data and you need to be prepared when (not if) something happens.
I have experienced a failure during a partition resize.
And do not use Partition Magic. It hasn't been updated for a long time and is not compatible with any OS released in the last 10 years.