The active partition is a primary partition used to boot operating system. A hard disk can have only one active partition.
To mark the current partition as active partition, click the button "Active" on the tool bar or click menu items "Partition" "Mark Partition as Active" in sequence; besides, right click the partition to be activated and on the popup menu select the item "Mark Partition as Active". If there is already one active partition, the following warning message will pop up:
Click "Yes", the current partition will be marked as active partition and active mark of the original one will be cleared.
The active mark of the current partition can be removed by clicking "Partition"--> "Remove Partition's Active Mark", which makes no active partition exist on the disk.