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External serial advanced technology attachment (eSATA) is a bus interface for connecting external storage devices. It is an extension of the serial advanced technology attachment (SATA or serial ATA) standard. It is designed to enable the SATA drive to be attached externally.
The SATA is a bus interface for connecting a host bus adapter to a storage device. The host bus adapter is an integrated circuit board having input/output (I/O) capabilities and a physical connector to a storage device or server.
The SATA was designed to replace the older advanced technology attachment (ATA) standard using the integrated drive electronics (IDE) interface and the newer version using the enhanced IDE (EIDE) interface. The enhanced version of ATA is the parallel ATA (PATA) with an EIDE interface. IDE and EIDE are also called ATA or PATA and are based on the IBM PC industry standard architecture (ISA). The biggest improved features are faster data transfer rate and hot swapping.
The standard interface for SATA is the advanced host controller Interface (AHCI). The AHCI has advanced features like input/output (I/O) operation procedures, quicker data transfer rate, native command queuing (NCU) and hot swapping. SATA will generally run in IDE emulation mode if the chipset or motherboard does not support AHCI. But IDE emulation mode cannot support advanced features. In 2004, the eSATA was standardized bringing external connection, an even faster data transfer rate and revised electrical requirements.
Other external storage devices are FireWire (or IEEE 1394) and the universal serial bus (USB). While eSATA is an older technology than the USB 3.0, it is still a competitor for data transfer rate. The eSATA does not need to translate data between the computer and the interface like the USB and FireWire. This added feature increases speed, reduces processor resources and does not need an additional off-load chip. But the eSATA does need its own power connector.