The max transfer speed in a computer is decided by what the SATA version is.
Max transfer speed from harddisk to motherboard is limited by how fast the disk can read or write, and how fast it can tell you what it read. The 'how fast it can tell you what it read' is capped by SATA speeds, but actual read and write speeds are much lower.
Compare this to someone dictating a story at 10 words per minute.
We have a typist who can type 30 words per minute. She can keep up without problems.
Now we get a typist who can type 60 words per minute. The story will still get created at 10 words per minute.
I have seen on many forums that buying a SATA 3 mobo is helpful only if you have to use USB 3 or an SSD.
To return to our story telling comparison. Lets compare a SSD to someone who can tell a story at 45 words per minute.
If we change the typist from the ones which typed up to 30 words per minute to the new 60 words per minute typist things will speed up. So it makes sense to buy a SATA-3 equipped motherboard is you will buy a SSD in the near future.
For USB3 it is a bit more complex. Consider this drawing:
| ------------ USB ------------ (External drive USB -> SATA HDD)
| | /-------- USB ------------ (External drive USB -> SATA HDD)
| | |
Notice that I drew three connections:
- One for the internal harddisk.
- One via a separate USB cable to external HDD 1.
- One via a separate USB cable to external HDD 2.
If both USB2 drives are on separate controllers and cables then both can sustain up to 35MB/sec. The internal drive can easily match that. Thus the limit would be the USB2 drives and not the internal HDD.
If both drives are on separate cables but are using the same USB controller (e.g. because they are connected via a USB hub. Either external or bone build into the motherboard) then the sum of both USB drives speed would be 35MB/sec. Each would do about 17½MB/sec. The slower part would still be USB, and not the internal drive.
If you changed the 'USB2 drives' for drives with a faster connection then things change.
E.g. if you use eSATA then most external (green) drives come quite close to the speed of the internal harddisk. With a single drive you would gain significant speed.
With two eSATA drives things become unpredictable. They may jointly max out the internal drive. Or, if you copy different files at the same time, things may actually slow down because the internal drives read gets less efficient due to a lot of extra head movements.
The same is true for USB3 connected drives, though USB3 has a disadvantage in due to extra overhead in the USB protocol.