dd(1) performance when copiing a disk to another

[Patrick Proniewski]

Hi,
(carte mère supermicro
chip SATA Intel 6300ESB)

I run FreeBSD 5.4 on a PIV 3GHz (SuperMicro motherboard, Intel SATA  
6300ESB chipset) with 2 SATA HDD. I'm in the process to duplicate the  
boot HDD to the second HDD. I run dd for that:

# dd if=/dev/ad4 of=/dev/ad6 bs=1m

It yields to poor performances:

$ iostat -dhKw 1
(...)
              ad4              ad6
   KB/t tps  MB/s   KB/t tps  MB/s
  124.49 252 30.69  128.00 246 30.69
  128.00 285 35.64  128.00 279 34.90
  128.00 282 35.27  128.00 283 35.40
(...)

Is it normal that data rate won't go upper than 35/38 MB/s ?

HDDs are: ad4 -> Maxtor 80 Go 7200 rpm
           ad6 -> Hitachi 80 Go 7200 rpm

one more question: is dd(1) a good way to duplicate a boot drive to  
make a bootable spare disk ?

patpro