To eSATA or not to eSATA, that is the question...

24 Jan 2009

eSATA is a technology to connect external hard disks directly to a computer's SATA port at the same speed as internal hard disks, instead of the more common USB connection at one sixths the speed.

As with most new and high-performance technologies, there are a number of issues surrounding eSATA, though.

I have been using eSATA for my own internal backup drive, as well as for several customer backup solutions.

First of all, keep in mind that on the electrical and logical level, eSATA is *identical* to SATA. The one and only difference between SATA and eSATA is the cable and connector. eSATA cables are shielded, since they are intended for use out in the open. The FCC requires this shielding to prevent broadcasting RF energy and interfering with radio devices. SATA cables are unshielded. The eSATA connector is I shaped (and has a connection for the shielding), the SATA connector is L shaped and doesn't.

This one difference has major consequences.

First of all, when you go out and buy the eSATA adapter, you need to be sure that it actually says eSATA - many vendors put a regular SATA connector into the external bracket of the PCI board to save a few pennies. This is actually illegal, a violation of FCC rules, because it doesn't have the shielding. Also, you need to make sure that the cable you are buying has eSATA connectors on both ends. You can buy cables with an eSATA connector on one end and a SATA connector on the other, to match these bad fake external SATA adapters. DON'T DO IT.

The next issue is eSATA-to-SATA brackets. Some external enclosures actually include them. Such a bracket simply is an eSATA connector in a slot cover, with an attached SATA cable that you plug into the motherboard.

Simple answer: don't use them. With the high frequencies going across the line, you will want one single unbroken cable directly from the port to the hard disk. At the connector in the middle, the signal gets reflected, and you get transmission errors.

I hear that you can sometimes make such problematic SATA connections work reliably by reducing the speed from 3 GB to 1.5 GB - but if you want the full 3 GB speed, you pretty much must use an eSATA adapter card.


OK, that's it for the hardware side. The next issue is hotplugging. Theoretically, eSATA can be hotplugged. So can SATA. In practice, that requires support from several components: the BIOS must support it. The hard disk itself must support it. And the operating system must support it.

The question, thus, is "Does Windows 2008 support SATA hotplugging?" I do not know the definite answer, but Vista definitely does support it. http://support.microsoft.com/kb/922976 describes an issue related to AHCI and hotplugging support. Another knowledge base article http://support.microsoft.com/kb/960735 mentions that Windows Server 2008 also supports AHCI, the specification that underlies SATA hotplugging. Incidentally, I have also seen eSATA drives listed under "safely remove hardware" on Windows XP, so presumably XP supports it, too.

Again, this will of course only work if both the hard disk and the motherboard/BIOS/PCI adapter support SATA hotplugging.