What Is External SATA (eSATA)?

Understand this external storage interface based on SATA standards

USB and FireWire have both been a huge boon to external storage, but their performance compared to desktop drives has always lagged. With the development of Serial ATA standards a new external storage format, external Serial ATA, has entered the marketplace.

USB and FireWire

Both USB and FireWire interfaces were designed as high-speed serial interfaces between a computer system and external peripherals. USB is more general and used for a wider range of peripherals such as keyboards, mice, scanners, and printers while FireWire is almost exclusively used as an external storage interface.

Black eSATA cable for connecting a hard drive to the motherboard, Computer component
mikroman6 / Getty Images

Even though these interfaces are used for external storage, the actual drives used in these devices still use the SATA interface. The external enclosure that houses the hard or optical drive uses a bridge that converts signals from the USB or FireWire interface into the SATA interface required by the drive. This translation leads to some degradation in the overall performance of the drive.

One of the big advantages that both of these interfaces implemented was the hot-swappable ability. Previous generations of storage interfaces typically did not support the ability to have drives dynamically added or removed from a system. This feature alone is what made the external storage market explode.

Another interesting feature that can be found with eSATA is the port multiplier. This allows a single eSATA connector to be used to connect an external eSATA chassis that provides multiple drives in an array. This can provide expandable storage in a single chassis and the ability to develop redundant storage via a RAID array.

eSATA vs. SATA

SATA and eSATA Cables
SATA (left) and eSATA (right) Cables.

External Serial ATA is a subset of the additional specifications for the Serial ATA interface standard. It is not a required function, but an extension that can be added to both controller and devices. In order for eSATA to properly function both connected devices must support the necessary SATA features; many early generation SATA controllers and drives do not support the Hot Plug capability that is critical for the function of the external interface.

Even though eSATA is part of the SATA interface specifications, it uses a different physical connector from the internal SATA connectors to better shield the high-speed serial lines transferring the signals against EMI interference. It also provides a 2-meter overall cable length compared to the 1 meter for internal cables. As a result, the two cables aren't interchangeable.

Speed Differences

One of the key advantages that eSATA offers over USB and FireWire is speed. While the other two incur overhead from converting the signal between the external interface and the internal based drives, SATA does not have this problem. Because SATA is the standard interface used on many new hard drives, a simple converter between the internal and external connectors is required in the housing. Thus, the external device should run at the same speed as an internal SATA drive.

The various interfaces each enjoy a theoretical maximum transfer speed:

  • USB 1.1: 15 Mbps
  • FireWire (1394a): 400 Mbps
  • USB 2.0: 480 Mbps
  • FireWire 800 (1394b): 800 Mbps
  • SATA 1.5: 1.5 Gbps
  • SATA 3.0: 3.0 Gbps
  • USB 3.0: 4.8 Gbps
  • USB 3.1: 10 Gbps

The newer USB standards are now faster in theory than the SATA interface that the drives in the external enclosures use. Because of the overhead of converting the signals, the newer USB still proves slightly slower but for most consumers, there is almost no difference. Accordingly, eSATA connectors are much less common now given that USB-based enclosures are much more convenient.