Copying Disk Drives with dd

[Janis Hamme]

Hello from Germany :)

I want to use the tool dd to create a backup of my hard disk. I have to 
identical sata2 hard disks.

My idea was just to copy them using
   dd if=/dev/sda of=/dev/sdb
but the speed was terrible slow. Not more than ~ 15 MB/s.

In order to find the problem I copied a 1 GB file to partition sdb8. 
Speed was about ~ 55 Mb/s. Seems good.
   dd if=/root/file of=/dev/sdb8

Next I tried to copy the same file to the same partition with the command
   dd seek=146818098 if=/root/file of=/dev/sdb
Speed was only ~ 15 MB/s although the result was the same.
It seems to be as slow as copying the whole disk.

I've made these tests with Knoppix and the Gentoo Live CD.

My Questions are:
   - Why is the speed that slow?
   - How can i speed it up without copying each partition.

Hope you are able to understand my problem (and my english).

Janis Hamme