doubt about iostat output

[Luca Ferrari]

Hi all,
I'm copying data between two USB disk, both ext4. Now the source hdd
provides 1MB/s throughput, as reported by hdparm tests, but the dest
hdd is writing at an higher speed. The thing is that monitoring with
iostat the copy I _always_ see at least a double speed of writing and
of transactions on the destination hdd. For instance:

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           3,52    0,00    1,01   50,00    0,00   45,48

Device:            tps    kB_read/s    kB_wrtn/s    kB_read    kB_wrtn

sdc              26,50         0,00      2970,00          0       5940
sdd              10,00      1024,00         0,00       2048          0

My doubt is: if the source hdd (sdd) is doing 10 tps), how can the
destination hdd (sdc) _always_ write the double? I don't get how and
where the data read is cached, since also the destination tps are
higher than the source one.

Thanks,
Luca