Do you use eSATA or USB3 to connect external disks to computers? Both are new. Which one works? Which one is the most practical?
eSATA is older than USB 3.0. Some external disk enclosures have eSATA connectors. Some computer motherboards have eSATA connectors. eSATA should win.
USB 3.0 is new. Almost nothing has USB 3.0 on board. The big difference is the large number of companies adding a token USB 3.0 port to their computer. USB 3.0 will win in a couple of years.
Thunderbolt will be here in a couple of years and make some uses of eSATA and USB obsolete.
Update 2012: USB3 won because the hardware is cheaper and USB3 is faster than current disks.
The current eSATA 2 is faster than USB 2.0 by such a large factor that copying a disk is practical. I copied a disk from an old computer to a new computer. Using USB 2.0, the copy would be overnight. I also tried to expand a computer by copying the current disks to external disks in USB 2.0 enclosures with a plan to then swap the disks. A 2 GigaByte disk required an overnight copy. The whole process chewed up two days. I made a similar change using eSATA and the whole process was about 5 hours.
USB 3.0 will be faster than eSATA 2 and USB 3.0 includes a decent supply of power in the same cable. USB 2.0 does supply some power to run small disks but not enough for desktop and server disks. USB 3.0 can supply the power for full size disks. USB 3.0 is faster than eSATA 2 and does not require a separate power cable. USB 3.0 will take over from eSATA 2.
The extra overheads of USB make USB 3.0 less efficient than eSATA and USB does not carry all the SATA signals carried by eSATA, making USB a bad choice for permanent storage. USB does not carry S.M.A.R.T. information. USB is inferior in many ways for frequent use. The one big advantage of USB 3.0 is the fact that USB handles a wider variety of hardware than just disks. Hardware manufacturers will use USB 3.0 for high speed cameras, scanners, printers, broadband network connections and disk to disk synchronisation. eSATA has only one use and will miss out when manufacturers are cutting costs.
Update 2012: S.M.A.R.T. information is now transferred through USB3.
eSATA 3 will kill USB 3.0 for disk connections because of extra speed and superior efficiency. When you backup your computer to an external disk, use eSATA 3 instead of USB 3.0. eSATA 3 is trailing USB 3.0 by about two years but one year is already gone. If your next hardware upgrade is in 2012, insist on at least one eSATA 3 connection.
Update 2012: All new motherboards and notebooks have multiple USB3 ports. Desktop motherboards might have one eSATA port, enough for a backup. New notebooks do not have eSATA.
Thunderbolt is a data connection aimed initially at replacing HDMI and DisplayPort. Thunderbolt will also work as a computer to computer data transfer connection. When Thunderbolt arrives in force around 2013, you can use Thunderbolt to connect two computers for synchronisation and backup. Thunderbolt is unlikely to connect a computer to an external disk. Thunderbolt will connect computers to storage devices. When you are copying terabytes of video from a RAID array on your computer to a RAID array on a backup computer, Thunderbolt will beat both eSATA 3 and USB 3.0.
So when do you use eSATA? When an external disk is connected long term. You have a desktop computer that is continually backed up by synchronisation to an external disk. You want speed and S.M.A.R.T. reliability reporting. eSATA is the only way.
When do you use USB 3.0? Give up on S.M.A.R.T. reporting and other features when you have to carry a disk from one computer to another. There is more USB 3.0 around than eSATA. USB 3.0 works with USB 2.0 when USB 3.0 is not available and USB 2.0 is everywhere. The only thing you need do is carry both USB 2.0 and USB 3.0 cables because they are different.
How do speeds compare to current disks? eSATA 2 and USB 3.0 handle the fastest magnetic disks with on or two per cable. You could use them to a RAID 1 array. The fastest SSD, Solid State Disk, is faster than eSATA 2 and requires eSATA 3. SSD id increasing in speed faster than magnetic disks and will soon match eSATA 3. If you want parallel backup to two external SSDs, you will need two eSATA 3 connections.
I am currently copying a disk with speeds varying from 28 MegaBytes per second up to 49 MBps. 49 MBps is about 490 Mbps (Megabits per second) or slightly faster than USB 2.0. The new disk is faster and the next upgrade, with a copy from the current disk, will require over 100 MBps and will still be within the range of eSATA 2.
The same computer has an SSD that can run at 275 MBps or nearly eSATA 2 speeds. In a year or two, when I upgrade again, the new SSD will be in the 500 MBps range, too fast for both eSATA 2 and USB 3.0. I will need eSATA 3.
One of my computers uses six disks in a RAID 5 array. Each disk can peak at close to 100 MBps at their outside edge where the data transfer is fastest. RAID 5 using 6 disks gives the transfer speed of 5 disks or 500 MBps. That is 5000 Mbps or close to the limit of eSATA 3. When I replace that storage oriented computer in a couple if years, the individual disks will peak at 150 MBps and require 750 MBps, or 7500 Mbps, for a parallel synchronisation, far more than eSATA 3 will handle. Hopefully Thunderbolt will be ready then.