On the mobile devices, especially for smartphones or PC tablets, people usually create a partition on their microSD card to help ease the burden of the internal storage on the device. Is that means if we create partition on SD card, it will increase the internal storage capacity? Of course not! On some mobile devices like Android powered device, there's some apps that could be used for moving the application data to the second partition of the microSD card, such as Link2SD apps. So, the application data that used to be saved on the internal storage could be moved to SD card, which it means user could save more space on the internal storage. As default, an SD card has only one partition with FAT32 file system, so find below link for step by step method of partitioning your sd card
http://tutorial-fornewbie.blogspot.com/2013/04/how-to-create-partition-on-sd-card.html?m=1
