cp /dev/hda3 /dev/hdb3

[smurspm1 at xs4all.nl]

I have a HD that was giving error messages and in general not
bootable/workable.

So I wanted to make a backup of the documents on that hard drive. I
entered this command:cp  /dev/hda3 /dev/hdb3. So far so good.

The cp command also asked this:cp: overwrite `/dev/hdb3'? I replied yes.

After about a hour I checked the progress. For some reason the command
that I entered copied the destination (hdb3) to the source hda3. Now you
understand that I'm not happy and I see no way of restoring the partition.

But I want to know what I did wrong?