[header] How to use your android as a portable hard drive [title] Buy a microsd card. [step] Only do this step if your phone has a microsd card slot for storage expansion. The memory card acts as the external memory so that the files aren't mixed with the folders that are essential for some of your apps to function. 
[substeps] Head to your favorite online store or computer/cell phone accessory store and purchase a microsd card. The maximum capacity depends on how much your phone can handle (flagship phones usually take up to 64 gb of expanded storage).