eSATA stands for External Serial Advanced Technology Attachment. When using a USB or Firewire external drive, you are actually using an ATA or Serial ATA hard drive, with an external enclosure. The Serial ATA interface is used for internal hard drives. It competes with universal serial bus (USB) 2.0 and also Firewire 800.
You will find that the practical data transfer rates of eSATA are higher than USB or Firewire ports. It has the same protocol as a SATA port and therefore, one can use the port with minimum modification. Another advantage is its low-level drive feature called S.M.A.R.T.
Connectors to these ports have lesser electromagnetic interference between devices, due to an extra layer of shielding. They aren't easily broken in general, and have a specific life of about 500 insertions and removals. The maximum cable length is about 2 meters (6.6 feet). The maximum speed for data transfer, actually realized, is about 150 Mbps or 200 Mbps.
eSATA Vs. SATA
eSATA is a subset of the additional specifications for SATA. It is not a necessary specification for your device, but one can add it as an extension to a controller, as well as other devices. Both the controller and device need to support SATA, in order for eSATA to work. This point is important, as the initial SATA controllers could not support hot-plugging facility. This is very important for the functioning of an external interface.
eSATA is a part of the interface specifications of SATA, but it uses physical connectors that are different from the internal SATA connectors. These connecters provide better shielding in case of high-speed serial lines, when transferring signals, providing better EMI protection. The eSATA cable length is 2 m, as compared to the 1 m length of internal cables.
eSATA Vs. USB
USB 2.0 is an easy-to-use, highly-compatible interface that is used on desktops, notebooks, and Windows or Apple servers. One disadvantage of USB 2.0 is that it is limited to a transfer speed of 480 Mbps and can translate into a maximum bandwidth of 30-35 Mbps, when using typical storage applications. USB 3.0, launched in early 2010, is said to have a theoretical maximum data transfer rate of 5 Gbps, which makes it ten times faster. It is also said to be full duplex, which means that it can download and upload simultaneously. eSATA speed, when compared to USB 3.0, is similar to the internal SATA 1.0 bus. This means it has a theoretical speed of about 3 Gbps. So, in conclusion USB 3.0 is faster than this port.
eSATA Vs. Firewire 800
eSATA is better, when compared with Firewire 800. Firewire is a built-in port on most of the Macs and provides good performance for various applications. You will find that even though eSATA may be running at slower speeds, than its actual potential, the overall performance would be better than Firewire 800. Firewire is 2.2 times faster, when it reads and writes.
One reason to go for an eSATA external hard drive is that it's inexpensive. You can even extend your SATA connectors with expansion cards that cost under USD 30. There is no translation involved in eSATA and therefore, it does not take long for the data to be transferred. This means the data transfer rate is fast, in the range of 1.5 Gbps to 3 Gbps.