I plan to by new core i7 laptop with eSATA 5400 rpm HDD and usb 3.0 and usb c ports.If I connect 500 GB Segate fast SSD (external), can I get practical best reed/ write speed of the SSD or will it have any limitations due to read/ write speed of HDD? Thanks.
Depends on what you are going to do with it.
When copying from the hard drive to the SSD, you will be limited to the hard drive speed.
When running applications from the SSD, the applications will most likely use the hard drive to store their application data files and cache, which can slow the applications down.
If you are just storing files on the drive to read from, then the hard drive should not affect the speed.