Sean Nelson
Forum Pro
A program can copy at full speed (i.e, the speed of the slower of the two drives) even though a transfer requires reading from one drive into memory and then writing to the other. It does this by using double buffering. Here's how it works:I'm guessing that a transfer from one external drive to another involves two steps: transfer from 'A' to memory, then transfer from memory to 'B'. This, in theory , would result in half the anticipated transfer rate.
- Read chunk 1 from drive 1
- Write chunk 1 to drive 2 while reading chunk 2 from drive 1
- Write chunk 2 to drive 2 while reading chunk 3 from drive 1
- Write chunk 3 to drive 2 while reading chunk 4 from drive 1
So you're keeping both drives busy at the same time.