The set active function is used to add or remove the 'active'
attribute, which means that the partition will be the one that
the computer should boot from by default (the partition that
contains the operating system). A PC will only boot the active
partition on the first hard disk. There can be only one active
partition on a disk, so setting one partition active will make
any other active partition inactive. It it also possible to have
no active partition if you are using a boot menu program (the
Partition Logic boot menu, or GRUB, LILO, etc.). If you are not
using a boot menu, then some partition on the first disk must be
active for the computer to boot. To set a partition active you
must first select it in either the partition diagram or the
partition list (when the selected item is empty space, this
button is disabled). No changes are made on disk until you
commit them using the write changes function.