You can use a hard drive or HDD to store computer files. It lets you save movies, photos, music, and documents, for example. You mainly use a hard disk to store lots of files. You can use an external hard drive as external memory for your computer or laptop if you want to safely store files with the intention of transferring them to another device. An internal hard drive must be installed inside of a computer or laptop. This is usually done to expand the storage capacity or replace a defective internal HDD, for example.