|The most limiting thing regarding max data transfer rates with an external drive is the interface between the external enclosure's circuits and the computer's circuits. |
The external enclosure should be rated to support USB 3.0 specs, or have eSATA connection capability, or both.
You get the fastest USB connected data transfer rates from a USB 3.0 connection / controller (USB 3.0 on both ends), but an eSATA connection is faster than that. An eSATA connection requires an eSATA port on whatever you're connecting to.
For a desktop computer, you can buy an internal SATA to eSATA wiring adapter that has a bracket with an eSATA port in it, that you install in a spare slot space. Some external drive enclosures that support an eSATA connection come with that, as well as an eSATA cable. .
USB 3.0 specs are backwards compatible with USB 2.0 and USB 1.x specs, so an external enclosure that supports USB 3.0 specs can be used with any computer that has a USB port.
Regarding the hard drive inside the external drive enclosure....
The max burst data transfer rate, e.g. 600 mbytes/sec, = 6.0 gbits/sec, for a SATA III drive (SATA drives transfer data at 10 bits per byte, not 8 bits per byte), can only be maintained in one continuous burst for very short periods of time, how long depending on the size of the ram cache on the drive's board.
Most of the time your hard drive isn't transferring data at anywhere near that fast.
When the hard drive is being accessed in one continuous burst, when the contents of the ram cache have been exhausted, the max data transfer rate the drive can obtain is the slower sustained data transfer rate.
What's more important is the maximum sustained (continuous) data transfer rate.
A solid state SATA III drive has a much faster sustained data transfer rating and yields you obvious faster access than a conventional SATA III drive, and many things run faster with a solid state drive, but they're relatively expensive per the capacity of them. The max burst data transfer rate of both types is the same.
eSATA is Still Faster Than USB 3.0
"Benchmarks show that the sustained data transfer rates are higher for FireWire than for USB 2.0, but lower than USB 3.0. "
Benchmarks yield real world speeds.
"...if you have a laptop an express card ssd. "
Cheaper laptops don't have an ExpressCard slot.
"The ExpressCard 2.0 standard was delivered March 4, 2009 at CeBIT in Hannover. It is expected to have a raw bandwidth of 5 Gbit/s (transfer speed 500 MB/s or 0.5 GB/s), which is ten times USB 2.0 (0.48 Gbit/s or 60 MB/s), and includes USB 3.0 (5.0 Gbit/s or 625 MB/s or 0.625 GB/s)."
Raw bandwidth = theoretical, not real world.
You can buy ExpressCard USB 3.0 controller adapters.
You can buy ExpressCard eSATA adapters that support SATA III specs.
StarTech.com 2 Port SATA 6 Gbps ExpressCard eSATA Controller Card (ECESAT32)
What is the actual data transfer rate for external hard drive enclosures?
See the lower part of the article.