Main battle tanks have internal and external storage space. Drobo is a manufacturer of a series of external storage devices for computers. It also includes an ethernet port, and eSATA and USB ports for attaching external storage. An exception is warehouse standing outside the system (external storage) with which the system communicates. Early laptops simply had no built-in high-speed interface for supporting an external storage device. A standard audio tape recorder can be used for external storage. Just like the SR-52, it has a magnetic card reader for external storage. Program written on personal computer can be easily copied and backed up on external storage. Depending on developer preferences, some apps can be installed to a phone's external storage card. The latter two do not need external storage and can be stored internally. In computing, external storage comprises devices that store information outside a computer. External storage can be connected using eSATA, Ethernet, USB, or a Secure Digital memory card.