For some devices/drives the limiting factor for their I/O can be limited more by their connection than by their nature. I.E. a NAS with a gigabit ethernet link may have more I/O speed than 1 gigabit.
In situations like this, it would be faster to copy the next file while verifying the previous one to take advantage of both sides of the connection at the same time. Copying the file eats up the 1 gigabit uplink and verifying eats up the 1 gigabit downlink, effectively doubling the speed in cases where the I/O is fast enough.
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